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TIGHT BINGING 
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OUP 786 13-6-75 10,000. 

OSMAN1A UNIVERSITY LIBRARY 

Call No. 5*9 Accewkm No. 1 

Author & 

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This book should be returned on or before die date last marked below. 




TH ' i.IV.!^ o PLANT 



A DESCRIPTION AND INTERPRETATION OF ITS 
FUNCTIONS AND STRUCTURE 



BY 
WILLIAM F. GANONG, Pu. D. 

PTIOFESSOU OF liOTANY TN SMITH COLUOGE 



LONDON 

CONSTABLE & COMPANY LTD 
1913 



COPYRIGHT, 1913, 

BY 
HENRY HOLT AND COMPANY 

Published April, 191:*. 



Of the scholars of Salomon's TTouso, "lastly, we have 
throe that raise the former discoveries by experiments, 
into greater observations, axioms, and aphorisms. These 
we call Interpreters of Nature." 

FHANCIS BACON, Tfie New Atlantis 



PREFACE 

The very first words I would write in this book are addressed 
to my botanical colleagues, whom I wish to inform that the work 
is not intended for them. In this statement I am by no means 
invoking immunity from scientific criticism, but emphasizing the 
aim of the book. It is not designed as a digest of our present 
scientific knowledge of plant physiology for the use of experts in 
that subject, but, in conformity with the aim of the series of which 
it is a part, it seeks to present to all who have interest to learn an 
accurate and vivid conception of the principal things in plant life. 
I was once myself such a learner, and I have tried to write such a 
book as I would then have delighted to read. It is, in a word, an 
attempt at that literature of interpretation which was fore- 
shadowed by Francis Bacon in the fine passage that stands on its 
dedicatory page. 

This aim will explain peculiarities of the work not otherwise 
obvious. Thus, I have been at more pains to be clear than to be 
brief, assuming on the part of my reader no great knowledge of 
the subject, but a large willingness to take trouble to learn; and as 
I have tried to discuss every process with fulness enough to eluci- 
date its nature, my book has wandered through a leisurely course 
to a length quite shockingly great. But I comfort myself with the 
reflection that the plan and the subject hardly permit other treat- 
ment; for a royal road to a real understanding of plant phenomena 
does neither exist nor can it be built. Perhaps, indeed, the very 
portliness of the volume will act as a deterrent to any attempt at 
a desultory reading in the hammock, and will rather suggest the 
study table, and the principal feature of an evening's business, 



VI 



Preface 



and sternly-preserved leisure for reflective concentration on the 
matters it considers. At least, any value it may have for the 
reader will be realized best through this mode of approach. 

As to the method of treatment in particular, I have sought 
especially to interpret those phenomena of plant life which come 
within ordinary observation and experience, penetrating just 
deeply enough into each to make clear the principle of its opera- 
tion, "the theory of the thing " in popular phrase; and some- 
times that has taken me far and sometimes it has not. Thus is 
explained the absence of some matters of high technical interest, 
which lie, however, outside the experience of the general observer. 
Where explanations are concerned, I have given the known ones 
when there are any, and when these are lacking I have not 
hesitated to supply suggestions of my own, though in a way 
designed to show their hypothetical character. As to statements 
of fact, I have meant to present only those which have acquired 
the impersonal validity of science, for which reason I have omitted 
a good many of the newest ideas, even at the risk of seeming not 
to know them; for I have noticed that he who is too closely up to 
date in science has later a good deal to unlearn. 

This deliberate conservatism is not, however, the inspiration 
of my advocacy of Darwinian adaptation, for that is based upon 
conviction as to its essential correctness. I am very well aware 
that some eminently respectable people now consider adaptation, 
except as an accident, an antiquated idea. I have myself expe- 
rienced periods of this belief, but have always found myself back 
to causative adaptation as the most rational explanation we 
possess of the relations of living beings to their environment. 
But while holding to the reality of adaptation as an historical and 
causative process, I do not by any means suppose that all plant 
phenomena are explainable on this basis; and in this book I have 
tried to sort out the numerous influences at work, and to show 
which phenomena are best explained by adaptation, which by 
mechanical causation, and which by others of the possible forma- 



Preface vii 

tive influences. But adaptation seems to me to guide the course 
of a mightier current upon which mechanical causation and other 
influences are ripples or eddies, or at least no more than the waves 
whose only lasting influence is occasionally to open new directions 
for the current to move in. With this belief in adaptation, I have 
naturally not hesitated to use the corresponding language of 
purpose, not a mystical, supernatural, forethoughtful purpose, 
but a physical, natural, experiential purpose, which does not 
presuppose any forethought, but only the preservation and 
accumulation of the results of past experiences wherein each step 
in advance was purely chanceful, and survived only because it 
happened to fit. 

There is one other matter of this kind I would mention, and 
that will be all. Throughout the book I have made great use of 
diagrams, generalizations, and conventionalizations; and this may 
seem inconsistent with the vitalistic rather than mechanistic tone 
of the work. The scientific and educational status of this practice 
are sufficiently explained in Chapter I, but I would like also to 
say that I think our advance in plant physiology is measured 
exactly by our ability to represent each detail in a mechanical 
diagram, a physical formula, or a chemical equation. For the 
evidence certainly indicates that every individual process of 
plants is purely mechanical, physical, or chemical. What cannot 
thus be explained, and what we have made as yet little progress 
towards explaining, is the nature of the influence which establishes 
and holds these processes in orderly sequences repeated in wonder- 
fully complicated cycles generation after generation. When we 
have explained the operation of each gun, and dynamo, and 
powder-hoist on a battleship, have we thereby explained the 
rationale of the operation of a battleship? Here is where the real 
difference lies today between mechanism and vitalism. And this 
is the vitalism of this book, not a supernatural vitalism of the 
theological type, and certainly not designed for theological needs, 
but a perfectly natural vitalism based on the superior interpretive 



Vlll 



Preface 



power of an hypothesis assuming the existence in Nature of an 
X-entity, additional to matter and energy but of the same cosmic 
rank as they, and manifesting itself to our senses only through 
its power to keep a certain quantity of matter and energy in the 
continuous orderly ferment we call life. If those complicated and 
regularly-recurring cycles of material and energy changes which 
constitute the visible phenomena of life were mechanistically 
self-originating, self-controlling, and self-surviving, then Nature 
should be full of scattered fragments of such cycles, whereas she 
is not. For everything in Nature has either all of the characteris- 
tics of life, or else it has none of them; it is either alive, or it is not. 
And there you have the chief argument of vitalism against 
mechanism. 

Having thus explained, the best that I can, the spirit and scope 
of this book, I turn to make my grateful acknowledgement to 
those who have rendered kind aid in its preparation. For the 
illustrations, in particular, I am indebted to many persons. For 
the privilege of using the two dozen or more fine pictures from 
Gray's Structural Botany' and the Chicago Textbook) as acknowl- 
edged with the cuts, I am indebted to the publishers of those 
works, the American Book Company; and I have also been per- 
mitted by the Doubleday Page Company to use figure 8, and by 
the Bullard Company to use figure 15, from publications of theirs. 
Further, a ready consent has been given by Professor G. F. Atkin- 
son to my use of figure 118, and by Dr. C. C. Curtis, to my use of 
figures 67 and 73, from books of theirs published by Messrs. Henry 
Holt and Company. In addition, I have copied a number of 
figures from various foreign works, notably those of Sachs, 
Kerner, Strasburger and Kny, taking pains, however, to acknowl- 
edge the sources with the cuts themselves. Further, I have made 
use without special acknowledgement of a good many pictures 
which have been copied so often as to have become a kind of 
common property (viz., figures 17, 35, 94, 147, 149 to 161, 164, 
166-7, 169-171), although these, together with certain others 



Preface ix 

whose source is acknowledged (viz., figures 81, 85, 107, 168, 175), 
have been re-drawn for this work by one of my students, Miss 
Bertha Bodwell, now Mrs. Richard Potter. The remainder of the 
pictures, somewhat over one-half of those in the book, are new. 
Several have been made by students of mine: figures 18 to 23, 
with 76 and 84 by Miss Bodwell: figures 27, 56, 57, 132, illustrat- 
ing physiological apparatus, with 126-7-8, showing phases of 
growth, by Miss Margaret Sargent: figures 103, 104, parts of a 
series representing the development of representative plants, by 
Miss Ruth Huntington, now Mrs. Max Brodel : figure 87 by Miss 
Stella Streeter: figure 133 by Miss Hope Sherman: while the fine 
graphs of figures 70 and 123 were worked out from the original 
materials as well as drawn by Miss Marion Pleasants. The photo- 
graph of figure 26 was given me by another student, Miss Anne 
Barrows, now Mrs. Walter Seelye. The elaborate and exact 
drawing of root tissues forming figure 53 was made by my col- 
league, Dr. F. Grace Smith, Associate Professor of Botany in 
Smith College, while the markedly original and very satisfactory 
series of generalized drawings in illustration of the principal 
physiological processes, embodied on the colored Plate I, and in 
the multiple figures 54, 66, 139, together with the figures 30 and 
99, were specially drawn for this book by another of my associates, 
Miss Helen A. Choate, Instructor in Botany in Smith College. 
To all of these willing and efficient collaborators I desire here to 
express my indebtedness, and my grateful thanks. The remainder 
of the illustrations, including the new photographs and diagrams, 
are productions of my own. 

But the greatest of my obligations is to Miss Choate, who has 
read both manuscript and proofs in a critical spirit no less militant 
because friendly. She has not been concerned so much with the 
scientific aspects of the chapters as with their exposition, rep- 
resenting in this the rights of the reader, for whose benefit she 
has curbed much exuberance of expression, and eliminated many 
an obscurity and inconsistency. That some of these faults re- 



x Preface 

main is not to be laid to her, since I have sometimes leaned back 
on superior official authority and had my own way. 

In the first announcement of the book it was said that keys, 
similar in principle to those used in works on classification, would 
be appended as aids to the reader in finding the explanations of 
phenomena. These keys, however, have assumed such propor- 
tions that it seems best to transfer them to a separate work. They 
are now in process of elaboration in detail by another of my 
associates, Miss Julia Paton, Fellow in Botany in Smith College, 
and will presently appear as a synoptical handbook. 

Finally, I recall that in advising the reader to try as many 
experiments as possible for himself, I said that practical guides 
to experimentation would be suggested in the Preface. Un- 
fortunately the one of these I consider the best, I am forbidden 
by modesty to name, excepting that I may mention, as our friend 
Mr. Dooley would put it in similar case, that it is entitled A 
Laboratory Course in Plant Physiology, is published by Messrs. 
Henry Holt and Company, and is written by myself. 

THE AUTHOR. 
Smith College, 

March 15, 1913. 



CONTENTS 



CHAPTER PAGE 

I. THE VARIOUS WAYS IN WHICH PLANTS APPEAL TO THE INTERESTS 

AND MIND OF MAN. (Methods of Study in the Science of Botany) . . 1 

II. THE PREVALENCE OF GREEN COLOR IN PLANTS, AND THE REASON WHY 

IT EXISTS. (Chlorophyll and Photosynthesis) 16 

III. THE PROFOUND EFFECT ON THE STRUCTURE OF PLANTS PRODUCED 

BY THE NEED FOR EXPOSURE TO LIGHT. (Morphology and Ecology 
of Leaves and Stems) 47 

IV. THE KINDS OF WORK THAT ARE DONE BY PLANTS, AND THE SOURCE 

OF THEIR POWER TO DO IT. (Respiration) 76 

V. THE VARIOUS SUBSTANCES MADE BY PLANTS, AND THE USES THEREOF 

TO THEM AND TO us. (Metabolism) 105 

VI. THE SUBSTANCE WHICH IS ALIVE IN PLANTS, AND ITS MANY REMARK- 
ABLE QUALITIES. (Protoplasm) 138 

VII. THE WAYS IN WHICH PLANTS DRAW INTO THEMSELVES THE VARIOUS 

MATERIALS THEY NEED. (Absorption; Roots) 165 

VIII. THE WAYS IN WHICH SUBSTANCES ARE TRANSPORTED THROUGH 

PLANTS, AND FINALLY REMOVED THEREFROM. (Transfer, Trans- 
piration, Excretion) 198 

IX. THE PECULIAR POWER POSSESSED BY PLANTS TO ADJUST THEIR 
INDIVIDUAL PARTS TO THEIR IMMEDIATE SURROUNDINGS. (7m- 

tability) 224 

X. THE VARIOUS WAYS IN WHICH PLANTS RESIST THE HOSTILE FORCES 

AROUND THEM. (Protection.) 256 

XI. THE WAYS IN WHICH PLANTS PERPETUATE THEIR KINDS, AND 

MULTIPLY THEMSELVES IN NUMBER. (Reproduction) 278 

XII. THE MANY REMARKABLE ARRANGEMENTS BY WHICH PLANTS SECURE 

UNION OF THE SEXES. (Cross-pollination; Flowers) 303 

XIII. THE WAYS IN WHICH PLANTS INCREASE IN SIZE, AND FORM THEIR 

VARIOUS PARTS. (Growth; physiological) 327 

XIV. THE ORDERLY CYCLES PURSUED IN GROWTH, AND THE REMARKABLE 

RESULTS OF DISTURBANCE THEREOF. (Growth; structural) 352 

XV. THE MANY REMARKABLE ARRANGEMENTS BY WHICH PLANTS SECURE 

CHANGE OF LOCATION. (Dissemination; Fruits) 378 

xi 



xii Contents 

CHAPTER PAGE 

XVI. THE METHOD OF ORIGIN OF NEW SPECIES AND STRUCTURES, AND THE 
CAUSES OF THEIR FITNESS TO THE PLACES THEY LIVE IN. (Evolution 

and Adaptation) 403 

XVII. THE REMARKABLE IMPROVEMENT MADE IN PLANTS BY MAN, AND THE 

WAY HE BRINGS IT ABOUT. (Plant breeding) 42( 

XVIII. THE PRINCIPAL GROUPS INTO WHICH PLANTS NATURALLY FALL, 

WHETHER BY RELATIONSHIP OR HABIT. (Classification) 44 

INDEX 46 



A TABLE DESIGNED TO DISPLAY THE PLAN OF THIS BOOK 



The description and 
interpretation of 
the Living Plant 
involves consid- 
eration of, 



The interests and 
capacity of the 
human mind in 
relation to the 
study of Plant 
Life, discussed in 
Chapter 

The nature and 
properties of liv- 
ing substance, 
called Proto- 
plasm, of plants, 
which, however, 
can be under- 
stood better after 
some study of 
the physiological 
processes, and 
hence is discussed 
in Chapter 6. 
Protoplasm. 



The physiological proc- 
esses of plants, con- 
cerned with, 



Nutrition, the pro- 
vision for daily 
physical needs, 
requiring, 



Maintenance of the 
Individual, de- 
pendent on, 



The methods by 
which plants be- 
come altered in 
structure, habits, 
and identity, in- 
cluding, 



The acquisition of food, which is constructed by 
plants inside their own tissues, as described in 
Chapter 

The development of photosynthetic structures, 
to which is devoted Chapter 

The release of energy, which supplies the power 
indispensable for every kind of work, aa shown 
in Chapter 

The transformation of food into special sub- 
stances needed for particular functions, as de- 
scribed in Chapter 

[A suitable place for the chapter which is logi- 
cally No. 2, as noted in column 2] 

The absorption of substances into the plant, 
with development of absorptive structures; 
hence Chapter 

The movement and removal of substances 
through and out of plants, considered in 
Chapter 

The adjustment of individual parts to surround- 
ings, to which is devoted Chapter 

The development of protective adaptations 
against hostile external conditions, discussed 
in Chapter 

The formation and development of new indi- 
viduals like those which produce them; hence 
Chapter 

The development of sex-uniting adaptations, se- ' 
curing the cooperation of two parents in pro- 
duction of offspring; Chapter 

The formation of new parts and their increase 
in size, to which is devoted Chapter 

The development of structures through cycles, 
both ontogenetic and climatic, Chapter .... 

The development of dispersive adaptations, se- 
curing room for new individuals to grow, as 

described in Chapter , 

In Nature, to which is devoted Chapter 16. 



Preservation of the 
Race, dependent 
on, 



Fitness to the sur- 
roundings, re- 
quiring, 



Replacement of old 
individuals, re- 
quiring, 



Attainment of adult 
size by new indi- 
viduals, requir- 
ing, 



8. 



9, 



10. 



11. 



12. 



13. 
14. 



15. 



The methods of alter- 
ation 



Chapters 

Methods 
Study 



of 



2. Photosyn- 

thesis 

3. Leaves and 

Stems 

4. Respiration 



5. Metabolism 

6. Protoplasm 



Under the hand of man, to which is devoted Chapter 17. 

The results attained, considered in Chapter 18. 



Absorption; 
Roots 

Transfer and 
Excretion 

Irritability 
Protection 

Reproduc- 
tion 

Cross-pol- 
lination; 
Flowers 

Growth, phy- 
siological 

Growth, 
structural 

Dissemina- 
tion; Fruits 

Evolution 

Plant Breed- 
ing 

Classification 




THE LIVING PLANT 

CHAPTER I 

THE VARIOUS WAYS IN WHICH PLANTS APPEAL TO THE 
INTERESTS AND MIND OF MAN 

Methods of Study in the Science of Botany 

ND he spake of trees, from the cedar tree that is in 
Lebanon even unto the hyssop that springeth out of 
the wall." Thus runs the record of the first botanical 
teacher, reputed also the wisest of men, as writ in the 
greatest of books. Arid from the days of King Solomon down 
to our own, men never have ceased to speak and learn of plants, 
until now the circle of knowledge has long been too vast for any 
one mind to encompass. To us, plants embrace not alone the 
cedar and the hyssop, but the fern, the moss, the lichen, the sea- 
weed, the mushroom, the mold, the blight, the yeast, and the 
germ of disease within the body of man. And it is not alone their 
forms, their uses, and their habits which concern us, but as well 
the minutest details of their internal construction: the mean- 
ings of their resemblances and their differences : the ways of their 
nutrition, increase, and adjustment to their surroundings: the 
possibilities of their development to greater and yet undiscovered 
utilities: and in truth no less than every fact which the intellect 
of man can discover about them. 

The field of botanical study is therefore not simply vast, it is 
practically limitless, in this respect transcending the natural 
powers of man, which are small. Therefore, while every school- 



2 The Living Plant 

boy can grasp the salient facts in that organized knowledge of 
plants which we call the Science of Botany, no one person can 
actually master any more than a limited portion thereof, es- 
pecially if he have the ambition to know it sufficiently well to 
aid in expanding the bounds of our knowledge. For the purpose 
of specialized study, accordingly, there have been developed 
within the science a number of divisions which are dependent 
on the nature of the problems presented, and therefore on the 
methods employed in their study. The divisions are these. First 
is 7cwsijj?c^ian (called also Systematic Botany, or Taxonomy), the 
oldest and most fundamental of all, and doubtless the theme of 
King Solomon's discourse. It establishes the relationships of 
plants to one another, and arranges them accordingly, while 
describing and naming them. It is studied through exact ob- 
servation and comparison of the external parts of plants, which 
can be kept preserved in a pressed and dried condition in col- 
lections called Herbaria, while its results are embodied not 
only in great monographs, but in handbooks, or Manuals, so 
arranged as to enable any person to identify plants for himself. 
Second is Morphology, which deals with the parts, or structures, 
of plants, and establishes their relationships to one another while 
describing and naming them. Morphology is very .-much the 
same to the parts of plants that classification.^ to plants as-^i 
whole. ^.The name in the past has been associated most closely 
with the comparative study of the large external structures, 
roots, stems, leaves, flowers, and fruits, and their transforma- 
tions into tendrils, spines, pitchers and the like, but is nowadays 
given a far wider extension; while special names describe the 
phases concerned with minute or internal parts, and needing the 
use of such exact and delicate instruments as the microscope 
and microtome, Embryology or ."IHe-histocy," for the develop- 
ment of the structures in the individual plant, Anatomy, for 
the cellular construction, and Cytology r^for the internal struc- 
ture of the cells themselves. Third is Physiology, a word which 



The Various Ways in Which Plants Appeal 3 

has precisely the same meaning with plants as with animals, 
comprehending the study of those functions or processes by which 
they secure the maintenance of their daily lives and the per- 
petuation of their kinds. It is studied chiefly through experiment 
by aid of the exact methods and instruments of physics and 
chemistry, though it reaches into realms which those sciences 
do not touch. Fourth is Ecohgy, youngest of the divisions of the 
science, and greater as yet in promise than performance, but 
nevertheless of the very first interest to a great many people. 
It explains the adaptations of plants and their parts, that is, 
the ways in which these are adjusted to the conditions of the 
world around, involving the meanings of their forms, sizes, 
colors and the like. This division has sometimes been called, 
and still is by 'some Germans, Biology; but that word should be 
kept for its legitimate use as meaning the study of life com- 
prehensively, and therefore equivalent to Zoology and Botany 
together. Fifth isj^an^^ Economic Botany), 

which is the study of the ways in which plants may be made to 
yield the greatest service to man. The older phases thereof, 
Agriculture i Horticulture, Pharmacology, and Forestry, originally 
purely practical, are now scientifically studied, and to their 
very great profit; while strictly scientific from their foundation 
have been the newer phases of Pathology , or the study of diseases, 
Bacteriology, or the study of germs and their effects, and Plant- 
breeding, or the systematic development of better kinds of plants. 
And to these divisions there is every promise that the near future 
will add yet a sixth, Botanical Education, which will attempt not 
only to train students much better in the science, but also to 
interpret botanical progress to the world at large. An important 
phase of this division will be the production of works, on the 
Natural History of Plants, which will set forth, with a combination 
of scientific accuracy and literary charm, not only the technical 
and economic aspects of plant life, but also those historical, 
legendary, and imaginative aspects which give to a study its 



4 The Living Plant 

widest human interest. Indeed, the production of such works 
may be viewed as the logical aim of all botanical study. 

Such are the principal divisions of botanical science as we 
know them at present. ThisJb&ok, concerned as it is with the 
life of plants, deals chiefly with** Physiology,' but the divisions 
are interlocked inextricably, and I must perforce make many 
an excursion into the others. This science, and all science, is a 
unit, and subdivisions thereof are nothing other than a concession 
to the limitations of the powers of man. 

As the reader reflects on this matter of the various divisions of 
botanical science, he cannot but notice how unequal they are in 
apparent utility to man, and he may even inquire why we should 
study at all the ones that seem useless. Two reasons at least 
exist why we should, and do. First, some people take pleasure 
therein, precisely as do others in art, music, and literature. No- 
body thinks of asking what use these latter may be, the value of 
pure pleasure being obvious enough; but the world has mostly 
yet to learn to extend the same approbation to the seemingly use- 
less sciences. Second, the history of human progress has shown 
that the greatest applications of science to the useful arts have 
sprung from purely scientific investigations of a non-useful type. 
Nothing, doubtless, could have seemed more useless to cotem- 
porary critics than the studies of those early naturalists who de- 
lighted to apply the new-made microscope to the investigation of 
the living atoms which swarm in slime; and yet from these very 
studies has come our knowledge of Bacteria, and our power to 
control the deadliest diseases that scourge mankind. Likewise 
photography, all the applications of electricity, a vast range of 
chemical arts, and indeed most others of the wonderful applica- 
tions of science to utility, have developed incidentally from purely 
abstract scientific researches made without any regard to useful 
applications. Furthermore, it is quite impossible to predict at 
what point upon the general surface of expanding knowledge the 
next useful discovery will spring forth. In fact there is no natural 



The Various Ways in Which Plants Appeal 5 

boundary between useful and useless knowledge; they are one 
and indivisible, and such boundary as may seem to exist is simply 
a shadow that shifts over the surface, changing with times and 
our customs. Accordingly, the only possible way in which human- 
ity can obtain useful results from science, lies through the en- 
couragement of the development of all of its phases; and this 
may be done with the assurance that now and then some useful 
applications will somewhere appear, and pay manyfold for it all. 
And this is precisely the reason, moreover, why no good system of 
education can confine itself to teaching useful knowledge alone. 
It is unfortunately still true, as it was when Stephen Hales, the 
founder of Plant Physiology, wrote nearly two centuries ago, that 
pure science needs protection "from the reproaches that the ig- 
norant are apt unreasonably to cast on researches of this kind, 
notwithstanding that they are the only solid and rational means 
whereby we may ever hope to make any real advance in the 
knowledge of Nature." When, therefore, the reader hears anyone 
asking what is the use of this or that phase of knowledge, or when 
he sees practical men showing impatience with the impractica- 
bility of great scholars and contempt for the usclessness of their 
knowledge, he may well state these facts by way of courteous 
reproof. And he may even add, as to such knowledge, that 
those who pursue it, in the absence of the material rewards 
reaped in full measure by practical men, deserve no less tribute 
of respect and approbation than is accorded by common consent 
to those whose efforts bring them personal wealth. Both in fact, 
though in different ways, are contributing to the welfare and 
progress of humanity. 

I have spoken, just now, of the pleasures of the study of Botany, 
and over this theme I would linger a little. It is true of all science 
that the pleasures of its study lie deep, and one must reach far 
before he can grasp them. It is not as with literature, for ex- 
ample, which makes appeal to the feelings, that lie near the surface 
and are easy to touch; for science appeals chiefly to reason, which 



6 The Living Plant 

lies deeper and is slower of action. This is why literature is en- 
joyed by nearly all people and science by only a few, and why 
literary reputations can be made in youth while those of science 
are mostly attained much later in life. Yet, when grasped, the 
pleasures of science are no less keen than those derived from any 
other field of intellectual endeavor, and I have even fancied that 
they yield an especially deep and lasting satisfaction, though in 
this perhaps I am wrong. There can be, I believe, no pleasure 
in life any greater than that which comes to the scientific man with 
the moment in which some truth heretofore not known to man- 
kind first dawns upon him; and it is in the hope of such moments 
of exaltation that he is willing to undergo toil, poverty, hardship, 
and even peril of life itself. The charm that there is in this pur- 
suit of truth receives many illustrations from the biographies of 
eminent scientific investigators, and especially from their familiar 
letters, in which can be seen more cleajty than elsewhere the 
actual workings of the scientific spirit.* But though felt to the 

* A characteristic example is furnished by the following letter written by Charles 
Darwin to Asa Gray, the eminent American Botanist. 

Down, August 9 [1862]. 

My dear Gray, It is late at night, and I am going to write briefly, and of course 
to beg a favour. 

The Mitchella very good, but pollen apparently equal-sized. I have just examined 
Hottonia, grand difference in pollen. Echium vulgare, a humbug, merely a case like 
Thymus. But I am almost stark staring mad over Lythrum; if 1 can prove what I 
fully believe; it is a grand case of TRIMOKPHISM, with three different pollens and three 
stigmas; I have castrated and fertilized above ninety flowers, trying all the eighteen 
distinct crosses which are possible within the limits of this one species! I cannot ex- 
plain, but I feel sure you would think it a grand case. I have been writing to Botan- 
ists to see if I can possibly get L. hyssopifolia, and it has just flashed on me that you 
might have Lythrum in North America, and I have looked to your Manual. For 
the love of heaven have a look at some of your species, and if you can get me seed, 
do; I want much to try species with few stamens, if they are dimorphic; Nesvea vert- 
idllata I should expect to be trimorphic. Seed! Seed! Seed! I should rather like 
seed of Mitchella. But oh, Lythrum! 

Your utterly mad friend, 
C. DARWIN. 

(Life and Letters of Charles Darwin, New York, 1888, II, 475.J 



The Various Ways in Which Plants Appeal 7 

fullest only by those who fare the farthest, the pleasures of science 
are by no means unknown even to youthful students; and I have 
myself experienced in the past and have since noticed in others, 
a keen enjoyment in the use of exact scientific methods and tools, 
a great satisfaction in the acquisition of knowledge that one feels 
to be solidly grounded, and a lasting pleasure in an understand- 
ing of the workings of the greater natural phenomena. But while 
the personal and aesthetic elements are certainly by no means 
absent from scientific study, as indeed the accompanying picture 
will bear witness, the student must realize that the deepest 
pleasures of science are of stern and spartan sort, somewhat 
like those felt by the strong man when he rejoiceth to run a 
race. 

We must return for a moment to the matter of the unity of 
botanical science in order to consider yet another concession, 
besides its artificial divisions, to human limitations. This unity 
of the science is of course but a reflection of the unity of Nature, 
where all of the vast number of facts and phenomena intergrade 
and interlock without any real boundaries. Yet the mind of 
man is so made that it can grasp only definite conceptions, and 
not many of these ; and it can no more form a definite image of the 
infinite intergradation of phenomena than it can of the infinite 
largeness of space or the infinite smallness of the sub-constitution 
of matter. Hence it is necessary, for purposes of education and 
exposition, to create definite images out of indefinite material. 
Take, as an example, the subject of leaves. Leaves are so many, 
so diverse, so intergradient, that no learner can grasp any con- 
siderable proportion of the facts about leaves as they actually 
are. The substitute therefor, to which every teacher and author 
is obliged to resort, is a subjective conception of a generalized 
or average leaf, built up for the learner from observation of a 
number of actual leaves; or, better, it is a composite conception 
of a leaf built up in the receptive mind of the learner from many 
observations of actual leaves, much as composite photographs 




,3 
o 

!3 



1 



The Various Ways in Which Plants Appeal 9 

of human faces are built up from exposures of many actual faces 
upon the sensitive photographic plate. This is precisely what our 
Text-books are doing when they devote chapters to "The Leaf/' 
"The Stem/' and the like. These titles do not represent things, 
but ideas; there are leaves in Nature but no such thing as the leaf. 
But the analogy of these composite conceptions to composite 
photographs goes yet a step farther, for, just as a real face is oc- 
casionally seen which resembles the composite face of the photo- 
graph, so an actual structure or phenomenon is sometimes found 
which is like our mental composite of its kind. Such a real thing is 
then said to be typical, and that is what is actually meant by this 
word in science. When, however, no typical representative of the 
composite is available, we are still not without resources; for it is 
possible to give exact and clear definition to the dim and elusive 
outlines of the composite itself by drawing firm sweeping lines 
through its more prominent places, a process which constitutes 
generalization, or conventionalization. When the data concerned 
are expressed in figures, then the result is a round-number aver- 
age, or conventional constant; when they are expressed in pictures, 
the results are generalized drawings, or, if simplified to mere struc- 
tural aids to the imagination, diagrams; when they are expressed 
in words, the results are generalizations, or verities, the "aphor- 
isms" of Bacon. Throughout this book, in accordance with its 
aim to interpret plant life in the large, I have made great use 
of composite conceptions, typical things, conventional constants, 
generalized drawings, diagrams and verities, to a degree which 
will meet with much disapprobation from my scientific colleagues. 
But I maintain that such generalized knowledge of plants is not 
only infinitely better than no knowledge at all, but is actually 
the most useful kind, as it is the only practicable kind, for the 
non-technical learner, whose knowledge in other departments 
of learning, in geography, history, and so forth, is largely of 
this character. And I further maintain that if only we would 
make greater use of it, along with its logically-correlated methods, 



io The Living Plant 

in our educational system, we should have less cause to complain 
of the comparatively empty condition of our elective science 
classrooms. It is not of course representative of the methods 
whereby scientific investigation is successfully pursued; but where 
else in human affairs do we insist upon teaching all people the 
technical methods or none? In large measure, Science, in order 
to be advanced, must be dehumanized; but in order to be used, 
it must be humanized. 

The fact is, the human mind is a very poor instrument for 
scientific research, for which it was never developed. Unless all 
of our knowledge is at fault, the mind of man was evolved under 
stress of use as his chief weapon in the struggle for physical ex- 
istence; naturally, therefore, all of its stronger traits are fitted 
to that very concrete activity rather than to uses of an abstract 
intellectual sort. Its power of concentration upon a single aim, 
with determination to achieve it by any means: its instinctive 
and partizan exaltation of its own case and minimization of its 
opponent's : its tendency to warp all testimony to its own credit : 
its quick defense of its own caste or clan, right or wrong, with its 
ready submission to the conventions thereof and contempt for 
everything outside: its preference for keeping to beaten and safe 
paths and for shunning the unknown, which it peoples with 
mysteries and evil designs : its liking for following the most assert- 
ive leaders and for leaning back upon their authority; all of 
these are invaluable traits in the struggle of the individuals of a 
social community for existence, but they form a very bad basis 
for scientific investigation, which requires the opposite qualities 
of disinterestedness, impartiality, and the judicial weighing of 
evidence for the determination of the exact truth without any 
regard to its effects upon persons, interests or dogmas. All men 
have the primitive self-centering qualities highly developed; and 
the scientific research of mankind is done upon a small residue 
of the opposite qualities which a few of them happen to possess, 
and which even in them are not so much natural as assiduously 



The Various Ways in Which Plants Appeal n 

cultivated. Is it any wonder, then, that scientific progress is so 
slow, so laborious, and so expensive? 

There remains one other phase of the relation existing between 
Science and the mind of Man, which is so fundamental to the 
subject of this book that we must give it some special attention. 
It concerns the apparent purposefulness of many biological 
phenomena, as expressed especially in adaptation. What, then, 
is this adaptation, with which the writings of Darwin have made 
us so familiar? It is any feature, whether of structure or action, 
which brings a life process into harmonious relation with the ex- 
ternal conditions that affect it. The flatness of a leaf is an adapta- 
tion to the need for a very wide spread of green tissue to light, 
as is to be fully explained in the following chapter. The colors, 
shapes, sizes and peculiarities of form in flowers are chiefly adapta- 
tions to the utilization of insects in the transfer of pollen, which 
is an indispensable prerequisite to cross fertilization, as will 
also be demonstrated in the suitable place. And other cases 
are known without number, involving not only single features, 
but often the cooperation of several. Now the question is this, 
in what way has this remarkable fitness of form to function, of 
structure to use, of parts to environments arisen? It was form- 
erly supposed that these adaptations were the direct work of the 
Creator, the ETERNAL, IMMEASURABLE, OMNISCIENT, and OM- 
NIPOTENT, as Linnaeus grandly characterizes him in the Systema 
Natures. But Darwin gave evidence, in The Origin of Species, 
greatest of all secular books, tending to show that they arose 
by a gradual process of evolution, developing in causative touch 
at every step with the conditions which they fit; and this view 
has long appealed as satisfactory to most biologists. But in 
our own day it is becoming somewhat customary to attribute 
adaptations rather to various adventitious origins, and to explain 
their persistence merely by the negative supposition that they 
are not out of harmony with the conditions concerned. In a book 
of this kind it is needful to take a definite position on this subject, 



12 The Living Plant 

if for no other reason than this, that the language one may use 
is concerned. My position in general is the Darwinian one, 
that adaptation in the main has arisen as a gradual causative 
accompaniment of evolution. Indeed, such a causative, or histor- 
ical development of adaptation appears to me an inseparable 
corollary of the very idea of evolution, and wholly independent 
of its method, whether it proceed by many imperceptibly small 
steps as Darwin believed, or by fewer and perceptible ones, as 
newer evidence seems to be showing. And the point about use 
of language is this, that if adaptation is a causative process, 
the feature developing in causal touch with the conditions con- 
cerned, then it is quite suitable and correct to say that the adap- 
tation exists for such-and-such a purpose; and I do not hesitate to 
use such expressions in this book. In so doing I am in the very 
best of company, for Darwin himself continually uses the language 
of purpose^ or teleology ; and both Huxley and Asa Gray, Darwin's 
devoted friends and co-believers, point out in their writings that 
evolution on the basis of Natural Selection places teleology on a 
scientific basis.* This fact is overlooked in our day by many, 
who think it scientific to avoid teleological or purposeful language 
as though it were a plague. Science, indeed, hath her fashiojis 
and her dogmas no less than other fields of human endeavor.*' 

A chief reason for the occasional denials of the causative origin 
of adaptation arises from reaction against the over-importance, 
and over-perfection, so often attributed to it. Adaptation has 
often been claimed on the scantiest evidence without any attempt 
at proof. At its best, however, adaptation can never be perfect, 
but is rather a general or generic affair, very much like our own 
adaptations to the trades or professions we follow. This is be- 
cause no feature of structure or function is free to respond to one 
adaptive need alone, but has to compromise with other consider- 

* An example of Darwin's teleological language is found in the passage from one 
of his books cited on page 234 of this volume. As to his establishment of teleology 
as a scientific principle, compare his Life ami Letters, New York, 1888, II, 430. 



The Various Ways in Which Plants Appeal 13 

ations which often have more influence than adaptation itself. 
Thus, in addition to the principal adaptation, (such for example 
as the flatness of a leaf in adaptation to the need for spreading 
much surface to the light), there are secondary adaptive needs, 
such as for protection against dryness or other hostile influences. 
Further, a prominent feature may not be adaptive, but incidental 
to some other process, as in autumn coloration of foliage, or the 
mathematically-arranged origins of leaves: or it may be merely 
a mechanical effect, like the drooping of old branches of evergreen 
trees: or it may represent an individual adjustment to one feature 
of the surroundings, like the bent-over leaf-stalks of house plants 
in windows : or it may be inherited from the past without present 
significance, as in the compound early leaves of the Boston Ivy: 
or it may represent a spontaneous new variation, or mutation, 
or sport, such as originate new garden varieties of flowers, leaves, 
or fruits; or it may have yet other meanings of minor sort. These 
cases and illustrations will all be further explained in the following 
pages, and I merely cite them to show that not all features of 
plants are adaptations, while all adaptations are interwoven more 
or less with these other considerations, the actual structure being 
the resultant of the interaction of them all. The matter can be 
expressed in this way, that adaptation can never fit a condition 
as an old glove fits the hand, but rather as a cloak fits the body. 
One should therefore neither expect too much of it on the one 
hand, nor reject it altogether on the other. The real problem is 
not so much to find adaptations as to separate out and define 
the various factors that enter into the combinations of which 
adaptation is only a part. 

One other important phase of the relations existing between the 
human mind and the workings of organic nature, concerns the 
question as to whether there is anything in living beings except 
physics and chemistry, in other words whether they are mechan- 
ism only, or whether the mechanism is inspired by vitalism. The 
evidence seems to be showing clearly enough that all of the in- 



14 The Living Plant 

dividual processes of plants and animals are purely physical or 
chemical, with no trace of a vital force in the old sense. Further- 
more, the orderly sequence and cooperation of these processes 
is largely explained by their linking up through the medium 
of stimuli, as will later be explained in the suitable places in this 
book. But it does not seem to me probable that the processes 
only happen to be thus linked up, or that these particular link- 
ings are merely the accidental survivors of innumerable ones that 
happened in the past. Indeed, the most reasonable explanation 
of the phenomena of organic nature in the large seems to me this, 
that all of the life processes are subordinate to some influence 
which is using living matter as a seat for its operations. Thus 
there would exist in nature not two, but three working entities, 
matter, energy, and this X-influence. Perhaps the living matter 
is the home which the principle of intelligence in Nature has 
built for its residence. This is something more than vitalism, 
or even the neo- vitalism of some philosophers; it is a super- 
vitalism. But its acceptance harmonizes some of the greatest 
difficulties in the interpretation of Nature, as the following pages 
will illustrate in the suitable places. 

Finally there remains one matter which I wish to add at this 
place. It may seem to the reader, as it will to some of my col- 
leagues, that in laying so much stress as I do upon causative 
adaptation, and a number of things of that sort, I am reading 
into Nature a principle closely akin to intelligence. If I seem 
to do this it is because that is my intention. I believe that the 
evidence now accumulating is sufficient to show that the same 
principle which actuates intelligence also actuates all the work- 
ings of Nature; or, as I have expressed the matter on a later page 
of this book, all living matter thinks, though only the portion 
thereof which enters into the brain of man is aware that it thinks. 
Our intelligence is a kind of epitomized expression of the prin- 
ciples underlying the operations of nature, very much as mathe- 
matics is an epitomized expression of the relations of number, 



The Various Ways in Which Plants Appeal 15 

or as the daily newspaper is an epitomized expression of the doings 
of civilization. And this I mean not as a metaphor, but as a 
serious scientific hypothesis. 

This discussion of adaptation and kindred matters, and per- 
haps some others of the matters contained in this chapter, will 
have little meaning, I know, to the reader who may be making 
his first acquaintance with plant life through this book. But I 
venture to hope that the case will be different after he has made 
some study of the pages which follow. Perhaps I should earlier 
have advised him to read this chapter the last ; and at least I do 
now suggest that he read it again after he has finished the rest 
of the book. 




CHAPTER II 

THE PREVALENCE OF GREEN COLOR IN PLANTS, AND 
THE REASON WHY IT EXISTS 

Chlorophyll and Photosynthesis 

manifold arc the works displayed in the world of 
living plants, that to one who seeks some tie to bind 
them all into a single natural group they seem at first to 
present only an endless diversity. They do in fact 
exhibit every possible gradation and variation; in size, from the 
stately Sequoia of the Sierras, or the giant Eucalyptus of Aus- 
tralia, towering high above all other living things and mighty in 
girth, down to the humblest weed of the wayside; inform, from 
the graceful tree with its spray of twigs and myriad leaves to the 
simplest sea-born plant whose life is wholly encompassed within a 
miniature globe : in color, from the quiet green of the forest to the 
brilliant hues of flowers, sea-mosses, or mushrooms: in texture, 
from the ivory-hard seeds of palms to the jelly-soft fronds of 
some seaweeds; in habit, from the independent life of the mightiest 
trees in the woods to the parasitic existence of a deadly germ of 
disease within the body of man. Nowhere among these features, 
nor yet among any others that we know, can we find a single 
one which applies to all plants. What is it then which binds all 
of this heterogeneous assemblage into a single natural group? 

Failing to find any one feature common to all kinds of plants, 
a scientifically-minded inquirer would next turn to ask what 
feature prevails most widely among them. If one marshals 
before his mental vision all of the great groups, from the flowering 
trees to the microscopical germs, and centers observation upon 

16 



The Prevalence of Green Color in Plants 17 

one after another, it gradually becomes plain that one feature, 
and only one, does prevail very widely, and that is the possession 
of green color. Moreover, a deeper study by aid of microscope and 
experiment shows that this truth is more nearly universal than 
appears at first sight, for a good many plants that display other 
colors, e. g., the red foliage plants of the gardens and the brown 
and red seaweeds, prove to be green in reality, though that 
color is masked by the presence of the others. 

But although the green color, which is that of a definite sub- 
stance called chlorophyll, is thus very wide spread among plants, 
there are some, nevertheless, which really do not have it. Such 
are the mushrooms, molds, mildews, yeasts and gerrns, as like- 
wise the Ghost Plant (or Indian Pipe), of the woods, the twining 
Dodder of the fields, and a few others. These plants are mostly 
white to brown, though they often exhibit very brilliant hues of 
red, yellow, and even a kind of a green, which, however, is very 
different in shade arid nature from chlorophyll. Ml of these 
brighter colors are easily removable by chemical means; and when 
that is done, the tissues are left either white or brown, with never 
a trace of the chlorophyll. 

There are, accordingly, plants which really are green and 
plants which really are not. And the reader's first natural 
thought, that so striking a difference in one feature is probably 
linked with differences in others, is correct. In the first place, 
observation at once shows a very fundamental difference between 
the two kinds in habit, for all of those lacking the chlorophyll 
are dependent for their food upon other beings, either upon liv- 
ing plants or animals, (in which case they are called parasites), 
or else upon their decaying remains, (when they are called 
saprophytes). In sharp contradistinction stand the green plants, 
practically all of which subsist without aid from other living 
things, thriving upon materials which they take from the air, 
the soil and the waters. A second great difference consists in 
this, that all of the non-green plants are small and of humble 



1 8 The Living Plant 

habit, as* the list above given will testify, contenting themselves 
with the odd and obscure places of nature, while the green plants 
grow grandly in stature and number, possessing the earth. And 
still a third difference exists, less likely to be thought of but no 
less important for our present inquiry, namely, the study of 
classification has shown that the non-green plants, for the most 
part at least, are descended in the course of a long evolution 
from green ancestors, and therefore have been green in the past. 
Hence we are brought to a generalization of the greatest impor- 
tance, the first indeed of the great botanical verities, the pos- 
session of chlorophyll is a well-nigh universal characteristic of 
plants, and their most distinctive feature. 

Such is the notable fact concerning the occurrence of chloro- 
phyll in nature. Obviously so wide-spread a substance must 
play some very great part in the life processes of plants, and it is 
our manifest duty to determine what it is. In any such study 
the first resort of the biologist, his first aid, as it were, to his 
ignorance, is observation, exact and interrogative observation, 
of so much as the eye can discover. If, now, the reader will look 
over, from this point of view, any collection of plants in garden or 
greenhouse, drawing meanwhile on his memory for additional 
facts from his own experience, he will find these things to be true; 
that chlorophyll is not omnipresent in those plants which pos- 
sess it, being absent from their roots and interior parts not reached 
by the light: that even in lighted parts it is not uniformly dis- 
tributed, being denser in the better-lighted places, as well ex- 
emplified in the deeper green of the upper as contrasted with the 
lower faces of leaves: that it does not develop at all in leaves 
which are grown out of the light, as witness the colorless sprouts 
of potatoes started in the darkness of cellars, or the grass of lawns 
accidentally left covered in spring: that it vanishes from green 
parts kept away some time from the light, as shown in the blanch- 
ing of celery when banked up with earth: and that most green 
parts turn over towards light when this comes rather strongly 



PLATE I 

Generalized drawings illustrating; the 
chlorophyll system of the plant 



PLATE i 




Sing. ^H from R D , Sin?le chlorophyl , 
grain from C. 



The Prevalence of Green Color in Plants 19 

from one side, as all plants kept in house windows attest. All 
of these facts unite to imply an extremely close relation between 
the meaning of chlorophyll to the plant and the action of light, 
even suggesting, indeed, that the chlorophyll is inserted, as it 
were, between the light and the use thereof by the plant. To 
this subject we shall later return, for we are dealing at present 
with the distribution of chlorophyll in the individual plant, a 
matter which can further be illustrated, in purely diagrammatic 
or conventional fashion, by the picture which forms figure A of 
Plate I of this book.* 

So important is chlorophyll, that the reader ought really to 
make its closer acquaintance through actual experiment ; for here, 
as everywhere else in science, an actual personal contact with 
facts or phenomena makes all the difference in the world in the 
clearness of one's understanding of them. It is possible to ex- 
tract the chlorophyll very easily from leaves. If one takes two 
or three soft thin green leaves, places them in any glass dish which 
is uninjured by heat, covers them with alcohol (of any of the com- 
mon kinds), and lowers the dish into hot water, then the chloro- 
phyll will come out into the alcohol before one's very eyes. Its 
most striking characteristic is the beautiful green color of the clear 
solution, together with a remarkable and beautiful red fluorescence 
which appears when the solution is held in some lights, and es- 
pecially when sunlight is focussed upon it with a lens. And the 

* This picture is meant to represent that which one would see on a surface ex- 
posed by a lengthwise cut through the center of such a reduced conventionalized 
plant. Such sections, called optical sections, are very much used in biological works. 
Thus, on the very same plate, (Plate I), appear optical sections of a piece of a leaf, 
a single cell, and a chlorophyll grain; and a good many others occur elsewhere in 
this book. In every case an optical section is supposed to be typical, that is, taken 
through the part most illustrative of the structure in question; and, where only one 
section of an object is given, it means that the object is substantially alike ail around 
the axis that is represented. Such sections, therefore, always stand for solid objects, 
and the reader should learn, as quickly as possible, to construct the solid in his mind 
from the section on the paper. This intellectual visualization, of course, requires 
imagination, but that is a quality which, despite the popular belief to the contrary, 
is highly essential to success in science. 



20 The Living Plant 

reader should experiment also upon its instability in sunlight, a fact 
of importance as will later be proven; this he may do by dividing 
his solution into two portions, of which he puts one in bright 
sunlight and awaits its changes of color, while he places the 
other in darkness for comparison. Incidentally, too, this experi- 
ment will show an important fact about the color of leaves apart 
from their coloring matters, for, when the action of the alcohol 
is complete, the leaves appear a soft creamy white. This, in 
fact, is the natural color of all living plant tissues when no special 
coloring material is present. 

We must, however, pursue a bit farther the study of the chloro- 
phyll substance, partly because of its importance, and partly 
because the study will lead the reader to an acquaintance with 
other matters which he should learn very early in his botanical 
studies. To the naked eye alone, no matter how closely applied, 
the chlorophyll seems to color uniformly the whole of the leaf, 
which, except for the veins, looks homogeneous in texture. But if 
we call to aid that wonderful instrument by which the range of the 
eye into the minute is increased a full thousandfold, that first 
and greatest tool of the biologist, the microscope, and place 
under its lenses a very thin section or slice cut right through some 
green leaf from surface to surface, then a very different idea of 
leaf structure is presented to the observer, as the accompanying 
picture attests (figure 2). And with this picture of an actual leaf, 
the reader should compare the generalized or conventionalized 
section represented in figure B on Plate I. Clearly, the interior 
of the leaf is not homogenous, but partitioned into a great many 
little compartments, with empty spaces here and there inter- 
spersed. These compartments are called cells, a word of vast 
importance in Biology, because not only the leaf, but all parts 
of all plants, and all parts of all animals, are composed of them. 
These cells differ greatly in details of structure according to their 
function, but are always compartments of some sort; and the 
reader should as promptly as possible incorporate this idea of 



The Prevalence of Green Color in Plants 



21 



universal cellular structure into his visual conception of plants. 
In our picture (figure 2), carefully drawn from an actual leaf, and as 
well in the conventionalized leaf (B on Plate I), the reader can 
see for himself the cells of the upper and lower skin (or epidermis), 
those of the vein (the clearer mass lacking chlorophyll), and 
finally those of the green tissue, distinguished by the large black 
or green spots which represent the chlorophyll grains. For the 




FIG. 2. A thin slice, or section, cut across a typical leaf (the European Beech), and highly 
magnified. From a wall-chart by L. Kny. In the original, the numerous black discs 
are green, as in the living leaf. 



chlorophyll really is contained in definite grains, and is not a dye 
spread all through the leaf. These cells are roughly spherical, 
cylindrical, or polygonal in shape, though the open clear air- 
spaces between them are most irregular in form. Each cell has 
its outer thin transparent wall (little more than a line in figure 2), 
within which comes a complete lining of a thin gelatinous sub- 
stance (shown in Plate I, J5, by the faint grayish or dotted 



22 The Living Plant 

shading), so nearly transparent as to be almost invisible. But 
though so insignificant in appearance, this grayish material 
is nevertheless the most important of all substances, for it is Proto- 
plasm, the exclusive seat and sole physical basis of all the phe- 
nomena of life, as I shall show in a later chapter devoted to that 
subject. Within this living substance, close up to the wall, lie 
the chlorophyll grains, each of which has a definite shape, some- 
thing like that of a disc or a lens, and consists of denser proto- 
plasm deeply stained by a green liquid which is the chlorophyll 
substance proper. Finally, it should be added, in order to com- 
plete the reader's conception of the cell, that all of the remainder 
of its interior is filled with the sap, which is simply water contain- 
ing many kinds of substances in solution. As to the spaces be- 
tween the cells, they contain as a rule nothing but air, which is 
in connection with the atmosphere outside of the plant through 
tiny little openings, called stomata, between the cells of the 
epidermis. We shall return, and that often, to this subject of 
cellular structure, and the reader will then recognize the ad- 
vantage of having thus made some preliminary acquaintance 
therewith. 

We must now return to the problem involved in the observa- 
tion that a close connection exists between the distribution of 
chlorophyll and the presence of light. Observation alone, how- 
ever, cannot lead any farther, and we must resort to the second 
of the biologist's methods, experiment. In such a situation 
the scientific mind would reason somewhat like this, if, as 
seems implied by the facts, the chlorophyll has in the plant a 
function dependent on the action of light, then some difference 
should develop between leaves kept for a time in darkness and 
others kept equally long in light. Accordingly the experimenter 
would darken certain leaves on a plant, in a way that would not 
injure their health, and then, after a day or two, would examine 
a darkened and lighted leaf side by side. The result is always 
disappointing to the naked eye, by which no differences at all 



The Prevalence of Green Color in Plants 23 

are discernible, but a very different story is told by the micro- 
scope. That indispensable instrument shows in the lighted leaves 
the presence of tiny white grains (figure D, Plate I), which are 
absent from the leaves that were darkened, while chemical tests 
prove these grains to consist of a definite and familiar chemical 
substance, starch. 

This fact that starch makes appearance in ordinary green 
leaves when exposed to the light but not in those kept in the dark, 
is so important in plant physiology that the reader should make 
some further and practical acquaintance with the matter. If he 
selects some one of the commoner house plants, (e. g., Fuchsia, 
Garden Nasturtium, Horseshoe Geranium), covers some of the 
leaves from the light by a box, exposes the plant for a day or 
two to light, removes the darkened and lighted leaves at the 
close of the second day, dips them for a moment into boiling water, 
blanches them of chlorophyll by aid of warm alcohol, immerses 
them in water a minute to neutralize the brittleness the alcohol 
causes, spreads them out in a white saucer, and covers them with 
a solution of iodine diluted from the tincture he may buy from 
a druggist, he will be rewarded by seeing a very remarkable 
difference develop between the lighted and darkened leaves, for 
immediately the former will all turn a very dark blue, while the 
latter will remain of their natural cream color. Now iodine, as 
anyone may prove by a touch to some part of his starched linen, 
though brown of itself turns starch a dark blue; and thus our 
experiment proves that the leaves form starch in the light but 
not in the dark. So exact, indeed, is this relation that if a famil- 
iar sharp pattern be cut in opaque material and applied during 
the experiment to the upper face of a leaf, that pattern is found 
reproduced in equivalent sharpness when the iodine test is ap- 
plied; and not only this, but if a photographic negative be used 
instead of the pattern, the picture will be printed very acciirately 
in starch in the leaf, and may be " developed " in remarkable 
fashion by the addition of iodine. For full success in these two 



24 The Living Plant 

latter experiments, however, special appliances and methods 
are necessary; and these are fully described in the various works 
devoted to experimental plant physiology, and mentioned in the 
preface to this book. 

If the reader should experiment at all widely upon this matter 
of starch formation in leaves, he will sooner or later come upon 
kinds which exhibit no starch whatsoever, even under perfect 
conditions of light. Chemical analysis, however, always shows 
this fact, that such leaves contain an equivalent amount of 
some sugar. Moreover, and this is a matter of consequence, 
analysis shows also that even the starch-forming leaves contain 
a sugar, and that, furthermore, it is from this same sugar the 
starch is made. We come therefore to a generalization of the 
greatest physiological consequence, the second, in fact, of the 
great botanical verities, and one which the reader should fix deep 
in his memory and incorporate with his visualized image of the 
working green plant, that plants containing chlorophyll make in 
the light a sugar which is commonly transformed into starch. The 
process being one of formation, or synthesis, under action of 
light, is called scientifically photosynthesis, while the substance 
made is the photosynthate. 

It will sooner or later occur to the reader to ask, especially 
if he has tried these experiments for himself, whether this photo- 
synthetic sugar is simply a transformation of something already 
existent in the plant, or a new substance that has been added 
thereto. This can be settled by the conclusive test of compara- 
tive weights; for, obviously, if it is a transformation, photo- 
synthesis would not be accompanied by increase in weight while 
if a new substance it would. It is with difficulty that I resist 
the temptation to describe to the reader the simple but highly 
satisfactory methods and instruments by which this important 
matter is experimentally determined; but my book has limits, 
and besides I am well aware that any attempt to exhaust my sub- 
ject is likely to produce a similar effect on my reader. So I must 



The Prevalence of Green Color in Plants 25 

simply state that the result of the test is perfectly conclusive, it 
shows that leaves, apart from varying amounts of water they con- 
tain, always gain weight in the light but not in the dark. They 
are always heavier in the evening than they were in the morn- 
ing. As to what becomes of the starch and sugar which disappear 



This square is ^ of a motor (a decimeter) on a side, and t J of a meter 
in area. 

An area of loaf exactly equal to this square would make iJ of a gram 
of grape sugar in an hour, or ^ of a gram in a day, or I gram in 
10 days, or 15 grams (which is i of an ounce) in a summer. 

This amount of grape sugar made in a summer, viz. 15 grains, would 
form a cube 2.15 centimeters on a side, the size of the small 
square in the lower right hand corner of this square. Or, it 
would form a layer over this entire square 1 millimeter ( 2 r K of 
an inch) thick, the thickness shown by the space between the 
larger and smaller squares. 




FIG. 3. Diagram to illustrate the quantity of photosynthate made per unit area 

of leaf. 

from the leaf, that will later be shown, though we may here note 
in passing that there is a continuous movement of the sugar from 
the leaves into the stem. Furthermore, this same method en- 
ables us to establish the amount of the increase in weight. This 
varies greatly, of course, with different plants and under different 



26 



The Living Plant 



conditions of light; but calculations have shown that for many 
plants collectively out of doors it approximates under average 
summer conditions to one gram for each square meter of leaf 
area per hour (scientifically expressed 1 </w 2 /i), or one twenty- 
fifth of an ounce per square yard per hour, and is about half that 




FIG. 4. Those cubes, which arc two-fifths the original size, show the amount of solid 
crystalline grape sugar made by a square meter (or yard) of leaf in an hour, a day, and 
a summer. 

amount in greenhouse plants in the winter. This figure con- 
stitutes one of those useful conventional constants which the 
reader should store in his mind, and keep ready for use. Ex- 
pressed in a different way, a leaf forms in a summer enough 
photosynthetic sugar to cover itself with a solid layer a millimeter 



The Prevalence of Green Color in Plants 27 

(one twenty-fifth of an inch) thick. The same quantities are 
also expressed in a graphic way in the accompanying figure 3, and 
still more expressively, perhaps, in figure 4. 

We must now examine more closely the photosynthetic sugar 
and starch which appear in lighted green leaves. The microscope 
does not show much about them, for the sugar is always dis- 
solved in the sap of the cells, and the starch, although solid, is in 
grains too small to be seen very clearly. Their chemistry, how- 
ever, is well-known and important. The sugar is of more than 
one kind, but the commonest is that known as grape sugar, 
or dextrose, which has the chemical composition, C 6 H 12 6 , and 
which is intermixed with some fruit sugar or fructrose having an 
identical formula. This formula, I need hardly say to the reader 
of this book, means that this sugar is composed of 6 parts of 
carbon, 12 of hydrogen and 6 of oxygen, though why this particu- 
lar combination of these three diverse elements should give a 
substance with the properties distinctive of grape sugar, nobody 
yet knows. Much less abundant in leaves is cane sugar, which 
has the composition C 12 H 22 O n . Starch has for its formula 
(ObHi O 6 )n, the n meaning a multiple, though for our purposes 
we may treat it simply as C H 10 O 5 . Now it is immediately 
obvious that these three substances, so closely associated in the 
leaves of plants, are also very closely related in their chemical 
composition, for they differ from one another only in their relative 
proportions of hydrogen and oxygen. Thus, 
Q>H 12 O 6 H 2 O = C H 10 O 5 

grape sugar water starch 

C 12 H 22 O n + H 2 O = 2partsC 6 H 12 6 

cane sugar water grape sugar and fruit sugar. 

C 6 H 10 O 5 + H 2 O = C 6 H 12 O 6 

starch water grape sugar 

2 parts C 6 H 12 O 6 H 2 O = C 12 H 22 O U 

grape sugar water cane sugar 

These three important substances thus differ, so far as their 



28 The Living Plant 

composition is concerned, simply in the proportions of the in- 
corporated water, though this tells by no means all of the story; 
but it helps to explain why they are so easily transformable by 
the plant one into the other. Taken together the facts suggest 
the probability that one of the three is a first-formed or basal 
substance from which the others are transformed. In a general 
way chemical research sustains this hypothesis, and points to 
grape sugar as the usual basal substance first formed in the light 
in green leaves. For all of our purposes, therefore, we may accept 
grape sugar as the conventional basal photosynthate, and its 
formula (C 6 H 12 6 ) should be fixed by the reader in his memory 
as another of the valuable conventional constants. 

It may seem to the reader just here that in treating this sugar so 
fully, I dwell overlong on a point of only subordinate value. But 
in this my critic would err, for, as a later chapter on the subject 
will show in detail, this photosynthetic grape sugar is the material 
from which, with certain transformations and some additions, 
plants make all of their substance and special materials, includ- 
ing their protoplasm, and derive all of their energy for work; in 
other words, it is their food. And since animals all take their 
sustenance, whether directly or indirectly, from plants, it is the 
basis of their food also. These facts may conveniently be brought 
together, even though somewhat in advance of all of the evidence, 
in this generalization, which constitutes another of the great 
botanical verities, that the photosynthetic grape sugar formed 
in green leaves in the light is the basal food of both plants and ani- 
mals. This sugar is therefore one of the three most important 
substances in organic nature, chlorophyll and protoplasm being 
the other two. 

Our next task is sufficiently obvious; we must find the source of 
supply of the materials entering into the composition of the sugar, 
which, the reader will remember, is an addition to the plant. 
Now a scrutiny, from this point of view, of its formula, viz., 
C 6 H 12 6 , at once reveals the suggestive fact that the H and the O 



The Prevalence of Green Color in Plants 29 

are present in exactly the proportions they exhibit in water, 
(H 2 0) ; this suggests that they may be derived from the water 
which, absorbed from the soil, always saturates the tissues of the 
living plant, and this hypothesis is confirmed by experiment. As 
to the carbon, a supply thereof exists both in mineral compounds 
in the soil, and also in the carbon dioxide, commonly called car- 
bonic acid gas, in the atmosphere. But experiment easily de- 
cides between these two sources, for when plants are grown in a 
soil or in water from which every trace of carbon is excluded, 
the plants make their photosynthate as readily as ever, thus ap- 
parently proving that the carbon must come from the air. At 
first sight it may seem an objection that this gas exists in the 
atmosphere in such an extreme of dilution, for it comprises only 
3 parts in 10,000, that is .03 (or ^) of 1 per cent. This amount 
is very small, it is true, though we must remember that the bulk 
of the whole atmosphere is vast in proportion to the bulk of all 
plants. However, suppositions cut small figure in comparison 
with facts; and it is easy to prove by simple experiments that 
leaves, or even small parts thereof, exposed to an atmosphere 
from which the carbon dioxide has been removed, can make no 
starch at all, although neighboring leaves or parts, exposed in 
the ordinary atmosphere, form it abundantly. Indeed, innumer- 
able facts unite to prove that the carbon used by leaves in the 
making of sugar is derived from the carbon dioxide (the carbonic 
acid gas), of the atmosphere. This, as the reader well knows, 
is the very same gas which is poured out by animals in breath- 
ing, by organic substances in decaying, and by fires in burning. 
The fact that leaves absorb this gas in making their sugar ex- 
plains in part the scientific basis of a widely known and very 
important phenomenon, that plants purify the air which is 
vitiated by animals. 

All chemical processes can be expressed in equations of the 
formulae of the substances concerned, and therefore we proceed 
to set down together the formulae of the carbon dioxide (viz., 



30 The Living Plant 

C0 2 ), and water, with the formula of the grape sugar they form, 
thus, 

In photosynthesis CO 2 and H 2 form C G H 12 6 

carbon dioxide water grape sugar 

Obviously now the proportions of the two former must be in- 
creased in order to yield the latter, thus, 

6 CO 2 + 6 H 2 O are needed to form C 6 H 12 O 6 

But a chemical equation must balance exactly on the two sides, 
and this in the present case can occur only thus, 

6 C0 2 + 6 H 2 = C H 12 + 6 2 

But such a balance of the equation implies that, in the making 
of sugar from carbon dioxide and water, oxygen is set free, and 
not only so, but in a volume exactly equal to that of the carbon 
dioxide absorbed. So striking a conclusion based upon purely 
theoretical evidence demands rigid test through observation or 
experiment. That a gas of some kind is released from green 
plants in the light is easily seen in submerged water plants which, 
if kept in an aquarium, give off tiny bubbles when lighted, though 
not in the dark; and everybody has seen those large gas bubbles 
which are caught in the felted green scum-plants floating on ponds. 
Analysis shows that the bubbles, in both cases, consist mainly of 
oxygen. But the matter can be tested much better by experiments. 
In a word, it is only necessary to place a green plant or a leaf in 
a suitable tight glass chamber, give it a known quantity of carbon 
dioxide (it has plenty of water), expose it for some time to the 
light, and then make a chemical analysis of the air in the chamber. 
The experiment yields an invariable result. A certain amount 
of the carbon dioxide has disappeared, and in its place there is 
present an exactly equivalent amount of pure oxygen. As to the 
significance thereof, it seems plain that the oxygen is a by- 
product formed incidentally in the chemical transformations, and 
useless in the main process. 



The Prevalence of Green Color in Plants 31 

Thus is our equation triumphantly vindicated, and we shall 
know it henceforth as the photosynthetic equation. Its importance 
and meaning may thus be expressed as another of our botanical 
verities, that the photosynthetic sugar made in green leaves in 
light is constructed from water drawn from the sail, and carbon di- 
oxide derived from the atmosphere, with an incidental release of pure 
oxygen, according to the photosynihetic equation 6 CO., 4 6 ///) - 
CaHuOo+60,. 

It may interest the reader now to know what quantities of 
these gases are necessary in the making of the sugar. For one 
gram thereof there are required 750 cubic ceiitimetQrs (about 
\ of a quart) of pure carbon dioxide, which is all that is con- 
tained in 2 cubic meters of atmosphere, and there is released the 
same quantity of pure oxygen. This, therefore, is the amount 
of those gases absorbed and released by a square meter (or yard) 
of green leaf each hour on a bright summer day. This release 
of oxygen, by the way, explains the remainder of the fact earlier 
mentioned, that plants purify the air which animals vitiate, for 
the plants not only remove the poisonous carbon dioxide from the 
air, but replace it by pure oxygen. And it may interest the reader 
to know how this balance of purification and vitiation works out 
between green leaves and men. Calculations have shown, in 
brief, that about 25 square meters (or yards) of green leaf are re- 
quired to balance the respiration of a man on an ordinary sum- 
mer day. But as the release of oxygen stops at night, it takes 
about 60" square meters of leaf working for a day to balance the 
man's respiration for 24 hours, and about 150 square meters work- 
ing through the summer to balance his respiration for a year. 

In composing the foregoing paragraphs I have given much care 
to the form of their presentation, for the reason that this particu- 
lar topic illustrates exceptionally well the principal method of 
scientific procedure in the acquisition of new knowledge. First, 
in the given problem, to observe all the facts that the militant 
eye can discover: next to compare and marshal the data thus won 



32 The Living Plant 

with a view to finding an explanatory principle: then to express the 
most probable conclusion in tentative form as an hypothesis: 
and finally to devise experiments whereby the truth or falsity 
of the hypothesis may be tested; these are the constituents of 
that scientific method through which all of our great scientific 
triumphs have been won. Hypothesis is a kind of a scout which 
Science sends on ahead to spy out the way for a further advance.* 
For the completion of our subject of photosynthesis, there re- 
mains but one matter of consequence, and that is the explanation 
of the association of light and chlorophyll with the process. We 
have seen earlier that the chlorophyll occupies a position between 
the light and the new-made starch or sugar, which fact implies 
that it forms a necessary link between the two. This in turn 
would suggest that the -chlorophyll perhaps acts on the light in a 
way to make it available for the photosyiithetic process. Tak- 
ing this hypothesis for guidance, we turn to investigate the effect 
that chlorophyll exerts upon the light which penetrates into it. 
Now the sunlight, as everybody knows, is a composite mixture 
of vari-colored lights, which, taken together, give the impression 
of whiteness. If this sunlight, however, be passed through 
chlorophyll, whether a living leaf or a solution in alcohol, there 
issues, as the reader will recall, only a clear green, or yellowish- 
green, light; and this fact seems to imply that all of the colors 

* That this is in practice, as it is in theory, the method of scientific men in their 
researches is illustrated by the following passage from the writings of the great 
German physiologist, Sachs. In connection with this very subject of starch forma- 
tion, he tells of his preliminary observations, on the basis of which, he says, " I 
came to the conclusion in 1862 that the enclosed starch, which had already been ob- 
served in the chlorophyll-corpuscles by Naegeli and Mohl, is to be regarded as the 
first evident product of assimilation [i. e., photosynthesis] formed by the decom- 
position of carbon dioxide. I said to myself, if this view is right, the formation of 
starch in the chlorophyll-corpuscles must cease on the exclusion of light, since the 
decomposition of carbon dioxide can then no longer take place; and that in like man- 
ner renewed access of light to the chlorophyll-corpuscles must also bring about a 
renewal of the formation of starch in them. These and similar deductions were con- 
firmed by appropriate investigations." (Lectures on the Physiology of Plants, 
Oxford, 1887, p. 307.) 



The Prevalence of Green Color in Plants 33 

in sunlight have been stopped by the chlorophyll excepting only 
the green. But the human eye is fur too crude an analyzer of 
color to be scientifically trustworthy, and we turn for aid to an 
instrument which science has devised for the exact analysis of 
light the spectroscope. I confess, it is only with reluctance that 
I refrain from explaining to the reader the principle of this beauti- 




FIG. 5. Diagrams to illustrate? analysis of light by the spectro- 
scope, a. Spectrum of pun* sunlight, h. Spectrum of sun- 
light passed through chlorophyll. 



ful instrument, one of the most delicate and exact, hut withal one 
of the simplest in theory, of all that have yet been evolved 
in the progress of science. It must suffice to say that the spectro- 
scope takes any mixture of colored lights, no matter in what 
complication, and, through the mediation of a prism, spreads 
them out in a band (called a spectrum), each color by itself. So, 
when a ray of white sunlight is sent into this instrument, it is 
made to fringe out into its red, orange, yellow, green, blue, in- 
digo and violet constituents, all beautifully clear and distinct, as 
shown diagramiriatically in our accompanying figure 5, a. Now 



34 The Living Plant 

if, while one is observing this spectrum, a solution of chlorophyll 
is inserted into the path of the light, a remarkable phenomenon 
follows, for the green liquid blots out from the spectrum most of 
the red and nearly all of the blue-indigo-violet, making those 
parts of the spectrum quite black, while all of the rest of the colors 
are left practically unaffected, as represented in our diagram 
(figure 5, 6). Chlorophyll, therefore, has power to absorb red 
and blue rays out of the sunlight, ignoring the others, in ob- 
serving which fact the active scientific mind would jump straight 
to the conclusion that these red and blue rays are probably the 
ones which are useful in photosynthesis. This hypothesis also 
is easily tested by experiment, for, obviously, if the red and blue 
rays really are those used in photosynthesis, while the others are 
not, then starch ought to be made under red light and blue light, 
but not under any others of the colors of the spectrum. It is 
possible to supply the different colored lights singly to the green 
leaf, either by use of colored glasses or liquids or by throwing a 
solar spectrum directly upon a leaf. The result of the experiment 
is conclusive; a leaf can form starch very readily under red light 
or blue light; but it can form none at all under the yellow, orange, 
or green. It seems a safe inference, therefore, that chlorophyll is 
a substance which picks out of white sunlight and applies to 
photosynthetic work, just those rays which can be utilized in the 
photosynthetic process, while rejecting the others; and all evidence 
attests the correctness of this conclusion. 

This conclusion, however, raises a correlated question, which is 
this, for what particular purpose is light needed in photosyn- 
thesis? Light, of course, is a form of energy, like heat and elec- 
tricity; and energy is the source of power which underlies every 
kind of work. Light, so physicists teach, consists of wave- 
motions in a space-pervading medium called the luminiferous 
ether; and the motion of these ether waves forms a source of 
power that can accomplish work, just as surely as can the billows 
of the ocean. Our problem, then, resolves itself into this, is 



The Prevalence of Green Color in Plants 35 

there in photosynthesis any step requiring the doing of work, 
and therefore the expenditure of energy? Our photosynthctic 
equation supplies the answer, for it shows that the oxygen set 
free has to be torn away from either the carbon or the hydrogen 
of the carbon dioxide or water, as a necessary preliminary to the 
union of the carbon with the remaining elements to form sugar; 
and other evidence shows that the carbon dioxide at least 
is thus dissociated. Now carbon dioxide is among the most 
stable of natural compounds, which means that its constituent 
atoms have an extremely strong affinity for one another, which 
means in turn that ample power must be exerted to tear them 
apart. Most people know that in our laboratories water can be 
separated into its constituent hydrogen and oxygen only through 
action of an electric current (electrolysis), or of intense heat; but 
carbon dioxide is even more difficult of dissociation. Here then, 
in the preliminary dissociation of this very refractory substance 
is that need for energy which we seek; and all the results of re- 
search confirm this conclusion. Why it should be the red and 
blue rays and no others which can do this work, we do not yet 
know, nor yet precisely the way in which the chlorophyll applies 
them to the task; but there is no question as to the facts. That is, 
chlorophyll is a transformer of light energy into photosynthetic 
work; and there you have the explanation of its function in 
plants, and the reason for its overwhelming prevalence in 
vegetation. 

We can now summarize this part of our subject as another of 
our botanical verities, the formation of photosynthetic sugar in 
leaves requires first the dissociation of the refractory carbon dioxide, 
which is effected by the energy of the red and blue rays of the sunlight, 
applied to that work by the chlorophyll. 

It will perhaps contribute further to clearness if we summarize 
the whole process of photosynthesis from another, and very human 
point of view. The formation of the photosynthetic sugar, the 
end of the whole process, is, after all, a manufacturing process 



36 The Living Plant 

comparable directly with those carried on by men, as the fol- 
lowing table well shows. 

The Factory The Leaf, or other green structure. 

Rooms therein The cells. 

The power Sunlight, the red and blue rays. 

The machinery Chlorophyll. 

The raw materials Carbon dioxide and water. 

The mamifactured product Grape Sugar. 

By-products Oxygen. 

The photosynthetic machinery can not only be apprehended, 
but also represented in a mechanical plan, as our accompanying 
diagram illustrates (figure 6). It represents the parts concerned 
in the process, (shown simplified in figure B on Plate I,) reduced 
each to a single one, and given a regular shape, though otherwise 
constructed and related as in the plant. Later we shall consider 
exactly the forces which keep the gases and liquids in motion 
in the suitable directions. 

The reader should now be able to visualize, or see vividly in 
imagination, this process in progress. Streaming in through the 
stomata and along the air passages is a steady current of the tiny 
particles, or molecules, of carbon dioxide, which reach the cell 
walls, pass in solution through these and the protoplasm into the 
chlorophyll grains, where they meet with water supplied in a 
continuous stream by the ducts. Here in the grain the chloro- 
phyll is stopping the red and blue light, and turning their vi- 
brating waves against the molecules of the carbon dioxide in a 
way to shatter that substance into its constituent atoms. The 
carbon thus forced apart from its own oxygen is uniting with 
the elements of the water into sugar, which is streaming into the 
sap cavity and then away through the sieve tubes, while the dis- 
carded oxygen is passing out from the grains through protoplasm 
and wall to the air space, along which it is streaming to the 
stomata and the outside world. And this is what is occurring 
inside of all leaves through all the bright days of the summer. 



The Prevalence of Green Color in Plants 



37 



So striking and far-reaching are the conclusions already reached 
in this chapter that anything added thereto must come as a kind 
of anti-climax; and therefore I wish we could stop just here. 
Moreover the chapter is al- 
ready over-long, though no 
longer, I maintain, than the 
relative importance of its 
subject sufficiently justifies, 
especially as it seemed to me 
best to make this first treat- 
ment of very important top- 
ics illustrate the methods 
through which our scientific 
knowledge has been gained. 
Yet several closely related 
matters, especially concern- 
ing the colors of plants, 
should have our attention 
before we depart from this 
subject, though I venture to 
suggest to the reader that he 
should not attempt to read 
all of this chapter at one 
sitting, but reserve the fol- 

07 t 

lowing part for a time by 

itselt. 

Ono nf tViPQP TYiattPTX mav 

une oi tnese matters may 

be dismissed VerV briefly. r 

^ 

Is it quite Clear tO the reader 

. - i_ if i i tions of movement. 

Why Chlorophyll lOOkS green Cells magnified uhout 200 and molecules about 




,.... f .. . . .. .. 

IG. 0. A diagram of the photosynthetic ma- 
chincry, showing tho parts reduced to the low- 
est possible terms, viz., a single living cell, with 
a single chlorophyll grain, a water-carrying 
(luct (on thc loft) and a sugar-carrying sieve- 

tubo (on tho rlght); shtulin|c is pro topiasm. 

^ 10 c ^ rc ^ es arc water; the squares are carbon 
dioxide; the triangles are oxygen; thc crosses 
are grape sugar; the arrows show the dircc- 



to the eye? This, indeed, 
is told very plainly by the spectroscope, when it shows that 
chlorophyll, in stopping the useful red and blue rays from 
the light of the sun, allows the other and useless rays to 



38 The Living Plant 

pass through as waste; and these of course are the ones which 
come to our eyes. Now these waste rays include the entire green 
light, which gives the principal color, together with all of the yel- 
low, which, mixing with the green, gives thereto the characteristic 
yellowish tinge which chlorophyll always shows. As to the re- 
maining rays, they happen to form complementary pairs; thus 
the bit of red and bit of green-blue form one pair, while the orange 
and unabsorbed blue form another; and as complementary colors 
(with lights) always give white or gray, these minor rays thus 
neutralize one another so far as color is concerned, and do not at 
all afl'ect the positive yellow-green. If it had happened that, in- 
stead of red and blue, the red and green had been the useful rays, 
then chlorophyll, and all vegetation, would have looked blue; and 
had green and blue been the useful kinds, then all vegetation 
would have looked red. The greenness of vegetation is simply 
the wastage of that part of the white light of the sun which is not 
needed in photosynthesis. 

In the early part of this chapter it was mentioned that many 
leaves of a red color really possess chlorophyll, which becomes 
visible when the red is removed by suitable solvents. This is 
true of the seaweeds, the red and brown colors of which are due 
to special pigments in the same grains with the chlorophyll; and 
there is good reason for believing that these colors bear a relation 
to the light conditions under which those seaweeds live, aiding 
the chlorophyll to utilize the sunlight as altered by its filtration 
through water. The case in the more familiar red plants of garden 
and field, however, is different. The colors in the foliage plants 
(Coleus, Copper Beeches, Japanese Maples) as well as in some 
vegetables (Beet, Red Cabbage), is a product of enormous in- 
tensification under cultivation; but in all cases the wild ancestors 
of these plants possessed some red color to begin with. This red, 
indeed, is fairly common in wild plants, where it shows especially 
in veins, petioles, nodes, or the under sides of leaves, and in the 
stigmas of many wind-pollinated flowers. It reaches, however, 



The Prevalence of Green Color in Plants 39 

its most striking, though a temporary, development in the young 
shoots of a good many plants (e. g., Maples, Oaks, and many 
herbs), which it flushes with translucent rose red as they push 
from the buds in the spring, though later it fades away with the 
increasing rapidity and vigor of growth. In all of these cases 
the color resides in a particular substance, named erythrophyll 
(or anthocyaij), dissolved in the sap of the cells, from which it 
can usually be extracted by hot water. It is typically a beautiful 
deep rose red material, varying much, however, in tint according 
to the conditions surrounding its formation, and the substances 
with which it is associated. Its identity, therefore, is plain enough, 
but concerning its significance to the plant there is very much 
doubt. One explanation argues thus; erythrophyll, as its color 
implies, permits the red rays of sunlight to pass unaltered, but 
cuts off, or at least weakens, the blue-violet ones. Now it is 
known that the red rays, while the most useful in photosynthesis, 
are harmless to the living protoplasm, but the blue-violet rays, 
though also useful in photosynthesis, are injurious, when un- 
tempered, to the living protoplasm and detrimental to some of 
the physiological processes; therefore (runs the argument), the 
erythrophyll probably acts as a protective screen, especially in 
the case of the early spring vegetation, admitting the beneficial 
red rays and tempering the noxious blue-violet rays until the 
formation of the chlorophyll, which, while developed for a differ- 
ent purpose, incidentally acts as a protection to the protoplasm, 
a subject to which, by the way, we shall return for fuller discussion 
in the later chapter on Protection. A second explanation is based 
upon the fact that erythrophyll has been found to possess a not- 
able power of transforming light into heat; it must therefore 
serve, this argument holds, to warm the tissues which possess 
it, and thus, during the bright but cool days of the spring, must 
facilitate those processes, such as nutrition, translocation of food, 
and growth, which are promoted by warmth. More recently a 
third explanation has been offered, based upon the fact that when- 



40 The Living Plant 

ever bright light, a relatively low temperature, much sugar, and 
some tannin happen to jcome together in a living cell, then the 
substance erythrophyll, of which the composition-color happens 
to be red, is formed incidentally as a purely passive chemical 
result. On this view the red color may be purely accidental, 
and may have no utility whatever to the plants which possess 
it, though the possibility is not thereby excluded that the plant 
may bring those conditions together, adaptively, in a cell where 
it has need for the red color to appear. The substance of the whole 
matter in reality is this, that we do not yet know surely the 
significance of erythrophyll in the plant; and herein lies another of 
the problems which make science so ever alluring. 

Connected with chlorophyll in a different way is one of the 
most striking and beautiful of all the phenomena of nature, the 
transition in the foliage each season from the uniform green of 
summer to the brilliant colors of autumn. Strangely enough, for 
a subject so important, our knowledge thereof is still very im- 
perfect, and there is even a difference of opinion as to the very 
significance of the colors to the plant. A basal fact, however, 
upon which there is agreement, is this, that the autumn color- 
ation results from changes connected with the death and fall of 
the leaf. We know that in late summer our trees are preparing 
for the annual leaf fall, in anticipation of which they are gradually 
bringing the activities of the leaves to a close, ceasing to make new 
chlorophyll, withdrawing certain precious materials into the stem, 
and building right across the bases of the leaves those corky layers 
which both cut them away from the stem, and also heal in ad- 
vance the wound that is thus to be made. Now chlorophyll, as 
the reader's own experiments will have shown, is soon destroyed 
by bright light; this destruction, indeed, is continually in progress 
throughout the summer in the living green leaves, where the color 
is maintained only through virtue of its constant renewal. It 
was formerly believed (and I mention the matter because the 
statement persists even yet in some writings), that this chloro- 



The Prevalence of Green Color in Plants 41 

phyll left in the leaf when the new supply ceases to form, breaks 
down in the light to other substances, which either themselves 
are highly colored, especially red, or else unite chemically with 
other materials in the cells to form colored compounds, the autumn 
colors being supposed, on this view, to be simply an incidental 
product of chlorophyll decay. But later research has shown this 
supposition to be wrong, for chlorophyll, in breaking down, does 
not form colors, but fades away to transparency in the leaf pre- 
cisely as it does in the alcoholic solution which the reader has 
placed in the sun. Now, sooner or later in the autumn the waning 
activity of the leaf reaches a point where no more chlorophyll 
is made, after which all of that substance already present fades 
away, with this notable result, that its disappearance renders 
visible any other colors which may have been present in the 
leaf, but masked by the greater brilliance of the green; and this 
fact constitutes the basal step in the explanation of autumn color- 
ation. As a matter of fact leaves do contain other coloring mat- 
ters, especially a bright yellow material, called xanthophyll, 
occurring in tiny grains associated with the chlorophyll. It is 
the exposure of this xanthophyll by the fading away of the chloro- 
phyll which gives the yellow, most common of the autumn colors, 
to autumn leaves. If the reader desires, he can himself extract 
this xanthophyll, and very easily, in a beautiful clear yellow solu- 
tion, by treating yellow autumn leaves precisely as he did the 
green leaves for extraction of chlorophyll, but using much leaf 
in proportion to the quantity of alcohol. Indeed the reader has 
seen the xanthophyll already, for, as he will recall, when he placed 
his solution of chlorophyll in the sun it faded away not to a trans- 
parent whiteness but to a clear yellow; this was xanthophyll, 
which itself fades away extremely slowly to whiteness. The 
whole situation must now be quite clear. Chlorophyll and xan- 
thophyll exist together in leaves, from which indeed they can be 
extracted and separated in beautiful solutions well known to 
all students in physiological laboratories; but xanthophyll is 



42 The Living Plant 

ordinarily completely masked by the far greater brightness of the 
chlorophyll, though it has influence enough to give the living 
leaf its yellow-green rather than a pure-green color. But xan- 
thophyll is vastly more resistent to the action of light than is 
chlorophyll, which explains its persistence in both leaves and 
solutions. The precise function of the xanthophyll, by the way, 
is not known, although it seems probable that this is to be found 
in some incidental chemical Connection with the chlorophyll, in 
which case its persistence in autumn leaves is purely incidental 
and of no service to them. 

Second in abundance, though first in brilliance, among autumn 
colors is red, which has a very different origin. It is due to the 
presence of that same erythrophyll which we have already con- 
sidered in connection with foliage plants and the spring coloration. 
This erythrophyll, also, the reader can extract for study in a beau- 
tiful clear rose-red solution by aid of the method he used for the 
chlorophyll, excepting that water must be used instead of alcohol, 
and the material should be abundant and consist of the very 
brightest red leaves he can find. Unlike the xanthophyll the 
erythrophyll is not present in the leaves before the chlorophyll 
fades away, at least not in appreciable amount; but it forms as 
the disappearance of the chlorophyll admits the light to the in- 
terior of the leaf cells. That the presence of bright light is es- 
sential to its formation is easily proven by experiment, and by 
the readily observable fact that in cases where one red autumn 
leaf overlaps another closely enough to shield it largely from light, 
the darkened portion is yellow not red; and this same fact further 
proves that red autumn leaves are actually yellow underneath the 
red. The brilliancy of the red, indeed, is proportional in general 
to the brightness of the light. But light alone is not sufficient 
to produce a formation of erythrophyll without the presence of 
the chemical substances requisite to its formation, which include 
certainly sugar and probably tannin; and it is only those leaves 
which happen to contain a sufficiency of these materials that can 



The Prevalence of Green Color in Plants 43 

turn red at all, the others being restricted to yellow. The Maples 
and the Oaks are trees well-known for their richness in sugar or 
tannin, which helps to explain why those particular trees are 
more brilliantly red than most others. It happens, furthermore, 
that erythrophyll formation, contrary to the usual rule with 
chemical processes, is promoted by lower temperature; and this 
explains why it is that a cool season promotes the brilliance of 
color, which indeed reaches its highest perfection in seasons or 
places where the skies are very bright and the frosts come 
early. 

Thus much for the facts as to the yellow and red autumn color- 
ation. We have now to take notice that two conflicting views 
exist as to its significance to plants. Many botanists believe that 
since erythrophyll seems to have definite functions in spring 
vegetation (as we have seen a few pages earlier), it has also an 
identical function in the leaves of the autumn, acting usefully 
as a selective light screen. The argument runs thus: chloro- 
phyll fades away in the leaf before the protoplasm has wholly 
ceased its activity: full exposure to bright sunlight, especially 
the untempered blue-violet rays, would injure this protoplasm, 
and act unfavorably on the translocation of the valuable materials 
from the leaf into the stem: an erythrophyll screen must temper 
the blue-violet rays while permitting the passage of the red rays 
which are not simply harmless but, being warm rays, would actually 
aid the final vital processes of the leaf during the cooling days of 
autumn. And those who hold this view assume that xanthophyll 
must have something of the same action, though inferior in degree 
to erythrophyll. On this view autumn colors are believed to be 
useful if not indispensable to the plants which possess them, and 
inferentially, have been developed adaptively to such use. 

Sharply contrasting, however, with this utility explanation of 
autumn coloration is the view that it is merely incidental. While 
the utility theory has certainly some facts in its favor, the most 
of the evidence seems to me heavily against it. Thus the utility 



44 The Living Plant 

theory, that of the protective and heating screen, requires in 
autumn leaves certain features which the spring coloration does 
in fact to some extent exhibit, viz., a prevalence of red rather 
than yellow, a fairly uniform coloration over all the parts to be 
protected or warmed, an especially deep coloration in the conduct- 
ing parts, and a fairly constant development of the color year after 
year without much regard to the details of the weather. As a 
matter of fact the phenomena of autumn coloration are differ- 
ent at almost every point red is less common than yellow; the 
colors are very uneven in distribution, forming spots, blotches, 
and streaks^ the color shows no particular tendency to cover the 
conducting veins: and its intensity varies greatly in different 
years, even almost to suppression of red in certain kinds of leaves 
in some seasons. The utility theory of autumn coloration re- 
ceives, therefore, no support from comparison with springfcolor- 
ation, even granting, as is not at all certain, that the latter is 
useful. The facts, therefore, taken all together seem to favor 
the incidental theory, which may thus be expressed ; that autumn 
coloration, for the most part at least, is a purely incidental result 
of the chemical and physical conditions which happen to prevail 
in ripening leaves and around them, and has in it no more element 
of utility than has the red of a sunset or the blue of the firmament. 
The yellow and red in the autumn coloration are so much more 
common and striking than any other colors that they naturally 
attract the most of our attention. Yet other colors occur, as 
everybody knows well, and as appears very clearly on the accom- 
panying plates (Plates II, III), which represent a selection from 
New England autumn vegetation, photographed in the natural 
colors. In fact, however, the great variegation thus displayed 
results from permutations and combinations of a very few colors. 
In addition to the red and yellow, there is only one other pigment 
at all common in autumn leaves, and that is an occasional brown, 
the mode of formation of which is uncertain. Most of the brown 
color in such leaves, however, belongs to the cell-walls, which are 



The Prevalence of Green Color in Plants 45 

white-transparent when alive, but turn brown on their death and 
decay. In fact the conditions prevailing in the ripening and dying 
leaf are most complex, for not only are different chemical sub- 
stances and physical forces interacting in large number, but their 
interrelations are constantly changing as the death of the proto- 
plasm weakens its regulatory control upon them. This combina- 
tion of complexity and changeability produces a state of unstable 
equilibrium, which permits even very minor external influences 
to exert relatively great effects, and thus is explained the differ- 
ences in the coloration of the same plants in different seasons or 
different places. In general, however, the effects of the weather 
upon the intensity of coloration are clear. Thus a bright autumn 
(and, equally, a sunny climate) intensifies the coloration, at least 
for the red, while dull weather is accompanied by dull coloration. 
Early frost helps somewhat to intensify color, partly by hastening 
the death of the leaf, and partly by aiding the chemical formation 
of the erythrophyll ; though frost is not, as many suppose, a cause 
of the coloration itself. Furthermore, the coloration can be 
brought on much earlier in the season than usual by any injury, 
a break in the bark, a split in the trunk, some damage to the 
roots, which weakens the vitality of the tree and hence pro- 
motes the waning of life in the leaves; and this is the explana- 
tion of the occasional Reddening of a single branch, or even 
whole tree, which one finds turning sometime ahead of its 
neighbors. 

The reader will feel, I am sure, that this is an unsatisfying answer 
to his natural wish for a definite knowledge of the causes of 
autumn coloration, but it is all that the present state of our 
knowledge permits. The subject has been studied heretofore 
by botanists from their side, and by chemists from theirs; but 
its problems will not be solved until some competent investiga- 
tor takes autumn coloration as his unit, and attacks it by any and 
all methods, chemical, physical, physiological, observational, 
experimental, or any others essential for attaining his ends. 



46 The Living Plant 

Some day this will be done, and then we shall know the meaning 
of autumn coloration just as surely as we now know the causes 
of the colors of chlorophyll, of fruits, and of flowers. Meantime, 
it is not the least of the pleasures of science that everywhere about 
us lie problems of moment, whose progress towards solution we 
may constantly watch, and the triumph of whose conquest we 
may perhaps even share. 



CHAPTER III 

THE PROFOUND EFFECT ON THE STRUCTURE OF PLANTS 
PRODUCED BY THE NEED FOR EXPOSURE TO LIGHT 

Morphology and Ecology of Leaves and Stems 




N the foregoing chapter we have considered photo- 
synthesis solely as a physiological process operating 
within the body of the plant, and have taken no thought 
for any relations it may have with the world outside. 
Yet the internal process is dependent on the external world in 
this very fundamental particular, that the supply of the indis- 
pensable light, carbon dioxide and water has to come from out- 
side. Furthermore, and this is a point of importance, the en- 
vironment rarely offers these essentials in precisely the right 
quantities, but sometimes too abundantly, oftener too sparsely, 
and sometimes in ways involving grave dangers. Their photo- 
synthetic needs plants cannot help, and their environmental 
conditions they cannot change, but there is one thing that is al- 
terable, and that is their own structure, with its large poten- 
tialities of adaptive development. Accordingly, in the course of 
long ages of slow evolution, plants have become so molded in 
form and in structure as to bring the photosynthetic process 
into advantageous or adaptive relation with the conditions of 
supply of the photosynthetic essentials outside, and in such man- 
ner, moreover, as to permit of particular adjustment to special 
peculiarities of the surroundings. Plants are like housekeepers 
who possess certain needs, and a desire for having the best, but 
who have no control over the purse-strings ; under the circumstances 
there is nothing for them to do but adjust the scale and style of 

47 



48 The Living Plant 

the establishment to the exigencies of a fixed income. This is 
the real meaning of the photosynthetic adaptations, which it is 
now our business to consider. Each one of the physiological 
processes of plants produces, of course, in like manner its 
effect upon their structure; but the one process of photosynthesis 
far surpasses all others, indeed all others put together, in 
the profundity of its influence in making plants what they 
actually are. The evidence thereof will appear in the following 
pages. 

The photosynthetic essentials for which plants are dependent 
upon the environment are in reality four, because, in addition to 
light, carbon dioxide, and water, plants need also, for reasons that 
will later appear, certain minerals, which are, however, for the 
most part very widely distributed in soils. Now in showing the 
way in which these four are supplied by the environment to plants, 
I must recall to the reader some very familiar and commonplace 
facts. But I remind him that there is nothing in the world so 
difficult to see in its real significance as the commonplace; more- 
over let him remember the truth expressed by a brilliant writer 
in the saying that little minds are interested in the extraordinary, 
but great minds in the commonplace. 

The crucial facts about the mode of supply of the four photo- 
synthetic essentials are these. 

First. They all exist widely even if not abundantly distributed in 
nature, and moreover are incessantly in movement or circulation, 
the light with the swing of the sun through the heavens, the car- 
bon dioxide with every breeze that stirs the still air, the water 
in the form of the mists and the rain, and the minerals in solution 
in the water which soaks and drains through the soil. Therefore 
plants have no need to go in search of these essentials, as animals 
must for their food, but are able to stay fixed in one place and 
allow the essentials to be brought them by the general circula- 
tion of nature. This method renders needless any self-motive 
power, with the accompanying muscular system and jointed skele- 



The Profound Effect on the Structure of Plants 49 

ton such as animals must have, and permits a simply continuous 
structure. This is why plants are sedentary beings, rooted for life 
in one spot. 

Second. The four essentials circulate in no definite paths or 
directions, but come to the plant from every point of the compass. 
This is true even of sunlight, despite the regular path of the sun 
through the heavens, for so uniform is the diffusion of the light 
through the sky that plants really receive it from every direction. 
And as to the wind, does it not blow where it listeth, and the 
waters, do they not cover the earth? Therefore plants have no 
need to face their parts in any particular direction, as animals 
must do in connection with their movements in search of their 
food, but face evenly outward in every direction, thus requiring a 
symmetrical distribution of their parts around a central vertical 
axis. This is why plants are radially built, presenting the same 
face to all points of the compass. 

Third. The four essentials are not evenly commingled, but seg- 
regated into two strata, the light and carbon dioxide in the at- 
mosphere above, and the water and minerals in the soil under- 
neath. Therefore plants must needs have two parts to their 
structure adapted to life in these two very different situations. 
This is why plants exhibit their primary division of structure into 
the green shoot (leaf and stem), and colorless root. 

Fourth. The four essentials exist rarely in abundance and then 
never for much of the time, and most commonly are sparser than 
plants can make use of. Frequently the light, always the carbon 
dioxide, often the water, and sometimes the minerals are accessi- 
ble only in dilution. Therefore the plant must needs reach out 
extensively to come into contact with a sufficiency, a condition in 
great contrast to that prevailing in animals with their concen- 
trated food and consequent compactness of body. This is why 
plants are branched so profusely and slenderly. 

Fifth. One of the four essentials, viz., light, is of such nature 
that it cannot be transmitted far into the plant, and therefore must be 



50 The Living Plant 

used at the surface. Hence plants have had to distribute the green 
tissues of the shoot in a manner ensuring the exposure of a great 
spread of surface to light, and this involves a flattening of most 
of the tissues of the shoot to the thinnest practicable structures. 
This is why leaves exist, and why the green plant consists of them so 
largely. 

Sixth. One of the essentials, the sunlight, falls upon plants from 
every direction in the aerial hemisphere. Not only does it come from 
a source which forever is changing its position in the skies, but, 
furthermore, this light is so strongly diffused through the atmos- 
phere that it falls upon plants from every direction in an in- 
tensity which for most of the time is as great as leaves can make 
use of; for it is a physiological fact that plants cannot use all the 
energy contained in full sunlight, and strong diffused light is 
enough for their needs. Hence it comes to pass that plants 
receive light in amount and direction sufficient to illuminate a 
great many leaves if only these are carried to various heights and 
spaced well apart, in a general distribution answering to that 
of the incident light. This necessitates the specialization of a 
part of the shoot for carrying the leaves upwards and outwards. 
This is the reason why stems exist and branch in such manner as 
typically to carry the leaves to a hemisphere of foliage. 

Thus it is evident that the most distinctive features of struc- 
ture and form displayed by plants of the highest development, 
the features indeed which are most closely associated with our 
very idea of plants, the sedentary habit, the radial symmetry, 
the diffuse-slender branching, the primary division into shoot 
and root, and of the shoot into flat leaves and supporting stems, 
all exist as adaptations which adjust the photosynthetic process 
to the conditions under which the photosynthetic essentials are 
supplied by the external world. It is therefore a fact that the 
photosynthetic process determines the ground form and primary 
structure of plants just as truly as it determines their ground 
color. 




The Profound Effect on the Structure of Plants 51 

It is worth while to try to express the sum of these features 
in diagrammatic form, and my suggestion thereof is contained 
in figure 7. The purely photosynthetic plant would exhibit 
a system of equal rigid branches 
springing as radii from a central 
trunk, and forking regularly 
outward to a vast number of 
young twigs which would turn 
up near the tips to spread the 
leaves horizontally in a hollow 
hemisphere of foliage. This 
theoretical form, of course, is 
modified in practice by other F. 7. Tin* form, as seen in vertical 

. i .. . 11 .1 section, which a plant would display 

Considerations, especially the (theoretically) if free to adapt itself to 

exigencies of mechanical sup- Ja^'K Further particu- 

port, as we shall later consider; 

but nevertheless it comes appreciably close to realization in the 
most typical of the great trees, when these are free to develop 
without interference, as was the case with the Oak of the ac- 
companying picture (figure 8). 

We turn now to a particular study of those two most distinctive 
plant structures, the leaf and the stem. A. first view over leaves 
in general gives only the impression of bewildering multiformity; 
but continued observation gradually sorts out the important 
from the trivial, arid builds one of those visualized composites of 
which I have spoken in the first chapter. As the reader should 
review and confirm for himself by inspection of a number of 
kinds brought together for the purpose, the principal part of an 
ordinary leaf is the spreading thin blade, which exhibits two con- 
stituents, first, the soft, seemingly-homogeneous, chlorophyllous 
tissue, denser in green on the uppermost surface, and seat of the 
food-making process, and second, the slender white veins, spring- 
ing out from the leaf-stalk and variously branching and inter- 
lacing while ever attenuizing towards the margin and tip of the 



52 The Living Plant 

blade. The tiniest veins are embedded within the green tissue, 
where they end in polygonal areas, as one can see with a lens in 
some leaves by holding them up to the light (for example in Rose, 
Cabbage, and Wild Ginger), and as shown in the accompanying 
cut (figure 9) ; but the larger veins stand out from the surface, 
though always from the undermost side where they are out of 
the way of the light. The veins have a double function, the 
conduction of water from the stem to the green tissue, and the 




FIG. 8. An oak troo, showing an approximation to the theoretical form of figure 7. 
(Copied from Hhuichan's American Garden.) 

conduction of the photosynthetic sugar back to the stem; and 
they have also a secondary use in helping a little to support the 
soft tissue, though the rigid but elastic stiffness of the healthy 
green leaf is due for the most part to osmotic turgescence, of 
which I shall speak in the suitable place. In addition to the blade, 
most leaves possess a leaf-stalk, or petiole, stem-like in appear- 
ance and function and varied in length, which carries the blade 
out into the light and aids to adjust it therein, as we shall later 



The Profound Effect on the Structure of Plants 53 

consider more fully under light-adjustment, or phototropism. 
Finally, some leaves exhibit, just where the petiole joins the stem, 
a pair of little leaf-like bodies called stipules, whose most remark- 
able feature is the diversity of their somewhat insignificant 
functions and forms. All of the parts of a typical leaf, blade, 
petiole and stipules, are well shown and in typical form, in the 
accompanying picture (figure 10). 





FIG. 9. A fragment of the vein system of a leaf, highly magnified, showing the typical 
mode of ultimate branching and ending of the vcinlcta. (From Sachs' Lectures, 
reduced.) 

FIG. 10. A typical leaf, the Quince. (From Gray's Text-books). 



The most striking of the features of leaves is perhaps the re- 
markable variety of their shapes, which seem in their multiform- 
ity to defy explanation or classification. Yet in reality the matter 
is simple, for there exist only three primary forms of which all 
the others are modifications and combinations, as the following 
analysis will show. 

First, the ideal condition for the best working of a leaf is ob- 
viously that in which it can have full exposure to all the light that 




Ilife^ 



FIG. 11. Leaves selected to illustrate the typical shapes; a photograph of living specimens, 

one-third the natural size. 



54 



The Profound Effect on the Structure of Plants 55 

there is, without any shading by its neighbors. This ideal ex- 
posure allows the development of the ideal type of construction, 
i. e., the shape that encompasses the most green tissue within 
the least outline, and a venation ensuring the shortest paths for 
conduction of water and the photosynthate. Such a leaf must be 
round, with its veins radiating from a central petiole. It is well- 
nigh realized in the leaf of the Common Garden Nasturtium 
(figure 11, c), a low-stemmed plant whose long petioles permit 
a full exposure of each leaf to light (figure 12) ; and it is shown con- 
ventionalized in figure 13, a. Furthermore, this association of 
round-radiate (or, in the current terminology, round-palmate), 
shape with full exposure to light is actually found in most plants 
which grow in such manner that their leaves do not shade one 
another, as for example in the floating leaves of Water Lilies 
(figure 11, a), Ground Ivy (figure 11, b), Wild Ginger, and others 
which trail or creep on the ground, and in low-growing long- 
petioled herbs like Geranium, Cyclamen, and Pelargonium, and 
partially in Ivies. Most of these leaves show a slit from the 
petiole to margin, but that does not alter the principle of the 
central-standing petiole, for the slit is merely a relic of the evolu- 
tion bf these leaves from kinds in which the petiole stood on the 
margin; indeed all intermediate gradations exist in heart-shaped, 
arrow-shaped, and "auriculate" leaves, where a part of the blade 
bulges backward on each side of the petiole. 

Second, the opposite extreme of habit is found where leaves 
are compelled to grow crowded together, as they arc in most plants 
living in especially dry or light places. In this case the best shape 
and arrangement would be necessarily the exact opposite of those 
found in the round type, that is, the leaves would be slender or 
linear, without distinction of petiole and blade, and with the veins 
running parallel; while they would take such positions as would 
admit the light most deeply and evenly among them, viz., they 
would point at the light and therefore stand parallel or radiating 
with respect to one another. Such a position for the leaves is in 



56 The Living Plant 

fact not at all bad for illumination, since diffused light can pen- 
etrate rather deeply among them, while the sun, in its daily swing 
through the heavens, slants its beams at times to the innermost 
parts of them all. The typical linear shape is actually realized 
in a great many leaves, of which our figure 1 1 shows a few (/, g, h) ; 
and it is shown conventionalized in figure 13, b. The association 
of linear shape with a crowding of leaves into dense-radiating 
heads is found typically developed in a good many plants, such as 




FIG. 12. The three types of plant form with which are associated the three fundamental 
types of leaf shape. On the left is the trailing Garden Nasturtium, in the middle, 
the half-desert ( 'ordyline, on the right the typical woods-plant Ficua reliaiosua. 

Spanish Bayonets, and the remarkable Tree Yucca of the deserts, 
in Century Plants, the ornamental Cordy lines (figure 12), and 
some of the Bunch Grasses. The association of the linear form 
with parallel-standing leaves is realized in the Flags and Cat- 
tails of stream margins, and especially in the Grasses of the 
meadows, which thus crowd a vast number of leaves into a lim- 
ited area. And another phase of the very same thing is presented 
by some of our evergreen trees, with their linear or needle-shaped 



The Profound Effect on the Structure of Plants 57 




leaves. These symmetri- 
cal cone-shaped trees may 
be viewed, indeed, as a se- 
ries of superposed meadows, 
spaced well apart in stories 
so arranged that each is 
smaller than the one next 
beneath it, thus avoiding 
injurious shading thereof, 
while the leaves point out- 
ward as well as upward to- 
wards the strongest light. 
This condition is repre- 
sented diagrammatically in 
figure 14, and it comes very 
close to actual realization 
in some of our Spruces and 
Firs when these are free to 
develop as they will (figure 
15). * This is the principal 
factor, I believe, in the ex- 
planation of the conical 
form of the evergreens. 

FIG. 13. Conventionalizations of 



Third, the conditions to 
which are adjusted the round 
and the linear shapes of 
leaves are uncommon in 
comparison with that in 
which numerous leaves are 
spaced at different heights 
along ascending stems, 
for this latter is the prevail- 
ing mode in vegetation, 
(figure 12, right). Since this 
condition is intermediate 
between the other two, we 
anticipate an intermediate 
shape of leaf, which would 
therefore be elliptical in out- 




the three fundamental types of leaf form. 



58 The Living Plant 

line with the petiole at one end and the veins branching off pin- 
nately from an axial mid-rib. This shape and venation are 
actually realized in the leaves of some trees, very typically in 
Chestnut (figure 11, d), Elm, Rubber-plant, and Banana. Much 

of tener, however, this outline is 
modified by a condensation of 
the green tissue towards the 
base of the leaf, which ensures 
a shorter path of conduction 
for water and the photosyn- 
thate, while lessening simul- 
* taneously the weight and lev- 

'w erage on the petiole. Such 
itzttt^ leaves are necessarily of ovate 
outline, and these ovate-pinnate 
leaves are very common in na- 
, , , , f .. . , . ture. The shape is well typi- 

riu. 14. The theoretical form, seen in ver- m 

tical section, of an evergreen tree. Further fied in the Catalpa, f Or CX- 
pnrticulurs in the text. /n * i \ t 

ample, (figure 11, e), and is 

represented in conventionalized form in our figure 13, c. In 
some plants the condensation goes so far as to make the leaf al- 
most round, as for example in the Red-bud (figure 11, i), when 
the venation makes some approach to the palmate type and the 
petiole is apt to be notably long. Such leaves often show a bulge 
of the tissue downward each side of the petiole, thus displaying 
a transition to the typical round shape with which we began. 

It is thus evident that three fundamentally-distinct condi- 
tions of leaf exposure exist, with three corresponding types 
of leaf shape, the rou^-radiate, the linear-parattel, and the 
or ?^"HH^!^- But innumerable intermediate conditions of leaf- 
habit exist, and therefore innumerable intermediate leaf shapes 
occur. These shapes have a large practical importance in the 
classification and description of plants, and accordingly have been 
named for this purpose with very great accuracy; and it is inter- 



The Profound Effect on the Structure of Plants 59 



esting to note that while some of the shapes have been named for 
their resemblance to familiar mathematical forms or common 
objects (e. g., ovate, lanceolate), the majority have to be desig- 
nated by combinations of these terms (as ovate-lanceolate, etc.). 

For completion of our subject 
of leaf shape, one matter of im- 
portance remains, and that con- 
cerns the curious emarginations, 
lobings, and compoundings which 
so many of the kinds exhibit. 
The margin of a leaf is typically 
smooth or entire, and many leaves 
actually exhibit this character; 
but others again are more or 
less waved, toothed, or incised, 
through the sagging, as it were, 
of the green tissue between the 
ends of the veins, or, occasionally, 
its swelling out beyond them. 
When this lobing becomes deep, 
it influences greatly the form of 
the leaf, especially as it follows 
the type of the veining. Thus, 
a deep lobing between palmate 
veins results in a shape like that 
of the Ivies, and the Maples 
(figure 11, j), while if it goes clear down to the leaf-stalk (in which 
case the separated segments usually develop little stalks of their 
own), it results in a leaf that is palinately compounded, like 
the Woodbine (figure 11, k). A similar deep lobing in pinnately- 
veined leaves leads through forms like those of the Oaks to 
pinnatj^^cgmgoun^ leaves, like those of the Locust (figure 
11, 1) and many Ferns, which latter, indeed, are often again lobed 
and compounded, and re-compounded again. In a general way, 




FIG. 15. Kngolmann's Spruce, showing 
an approximation to the theoretical 
form of figure 14. (Copied from Kirkc- 
gaarcl's Practical Handbook of Trees, 
etc.) 



60 The Living Plant 

as will later appear, there is a probable adaptational advantage 
in the compounding of leaves, since it aids them to resist the 
tearing action of strong winds, and there is a possible adaptive 
explanation of the deep lobing of leaves like Ivies and Maples 
in the opportunity thus afforded for an interlocking of the leaves 
and consequent utilization of every ray of the incident light. 
But nobody, so far as I can find, has yet been able to give a reason- 
able explanation of the significance of the emarginations of leaves, 
for the suggestion that the points thus resulting serve to collect 
atmospheric electricity for some use by the leaf can hardly be 
seriously entertained. Emargination, lobing and compounding 
are evidently three degrees of the same thing, but it is by no means 
necessary to believe that because compounding is adaptively 
Useful, therefore emargination must be useful likewise. On the 
contrary, it is not only possible that the emargination of leaves 
originates non-adaptively in some manner purely incidental 
or accidental, and is later intensified adaptively to lobing and 
compounding, but the method embodied in this supposition affords 
the most reasonable explanation we yet possess of the origin of 
adaptations. 

While adaptation to the mode of exposure to light is the chief fac- 
tor in determining the shape of the leaf, other adaptations and influ- 
ences, very different in different cases, exert also their effects, 
making the shape of any given leaf a resultant of the cooperation of 
many influences. This fact the reader must remember when he 
tries to apply the principles of the preceding pages to the ex- 
planation of leaf shapes he may find in his walks abroad in the 
country. At first he will find so many exceptions and contra- 
dictions that he may incline to dismiss my explanations as ground- 
less; but if he will continue his observations with patience, he 
will gradually find the exceptions disappearing and the essentials 
standing out in those composite conceptions of which I have 
spoken in the first chapter; and then, I believe, he will agree 
with the conclusions here expressed. 



The Profound Effect on the Structure of Plants 61 

From the leaf we turn to the associated and well-nigh equally 
distinctive part, the stem, of which, however, the structure is 
comparatively simple and uniform. Since its principal function 
consists in raising and spreading a great many leaves to the light, 
it must of course be adapted to provide a firm mechanical support 
in conjunction with much branching; and in fact it consists of 
a cylindrical-tapering, rigid-continuous, regularly-ramifying struc- 
ture familiar in the stems of the majority of plants. Although older 
stems become strongly thickened and woody, and protectively 
enwrapped in layers of bark, the young growth is soft and green 
like the leaf, and likewise consists of veins and soft tissue, though 
the relative importance of the two is reversed in the stem as com- 
pared with the leaf. The veins can be seen by the eye in young 
stems that are translucent (e. g., Balsam), when these are held 
to the light; and they can also be made visible through the tissue 
in some others if these are stood with their cut ends in a deeply- 
colored liquid. And they can always be seen in thin sections cut 
crosswise of the stem, as well illustrated in some later figures 
(73, 139, J5) which accompany a fuller discussion of the stem 
in another connection. The veins form a ring in most kinds 
of young stems, though in some they are scattered about; and 
wherever they branch to run out to the leaves the stem is commonly 
swollen a little, and oftentimes lighter in color, giving origin to 
the so-called nodes separated by spaces called internodes, which 
are by no means "joints," as sometimes described. Outside 
the ring of the veins, as the later figures 73 and 141 show very 
clearly, the soft tissue holds chlorophyll, and thus aids the leaves 
in their photosynthetic function. The amount of such work 
that stems can do must in fact be little; but the plant takes ad- 
vantage, as it were, of every bit of its surface exposed to the light 
and not needed for other uses, even including such parts as the 
stamens and pistil of the flower, to spread out additional chloro- 
phyll for the invaluable photosynthesis. 

Stems, as a rule, grow continuously from buds at their tips, 



62 The Living Plant 

and new branches from buds in the angles between stems and 
leaves, a position which has the advantage of nearness to the 
manufactories of food. This brings us to consider the causes which 
determine the arrangement of leaves on the stem, a curious matter, 
scientifically called phyllotaxy, ^hd once discussed more commonly 
than now in botanical books. ''I/eaves do not originate on the stem 
at hap-hazard, as may seem the case on some slender branches, 
but in quite definite and even mathematical order, as rosette- 
like plants, cones, and some other very compact structures sug- 
gest. Two primary systems of leaf-arrangement are possible, 
and occur. The simplest is the opposite (or whorled) system, in 
which two leaves stand at the same node exactly opposite one 
another, as occurs for example in the Mints, (figure 16, A), in 
which case the next pairs above and below stand at right angles 
and thus cover the space left by the first set, producing four vertical 
rows often in remarkable symmetry, as our common cultivated 
Coleus illustrates. This, with the other arrangements, is shown 
diagrammatically in figure 16, where the reader is supposed to 
look down from above on the stem, which is imagined to be tel- 
escoped, so to speak, Chinese lantern fashion, to a single flat plane, 
as indeed the stems actually are in the buds. In some kinds, 
three instead of two leaves stand at a node, or four or five, or 
more, producing a regular whorl, but in all such cases, illustrated 
for instance by large Lilies (figure 16, J5), the leaves in a whorl 
are evenly spaced and cover the breaks in the whorls above and 
below. This is the system prevalent in flowers, for, as everyone 
will recall, the whorl of sepals covers the breaks in the whorl of 
petals, with a similar arrangement in stamens and carpels. Thus 
much for the opposite or whorled system; the other is the spiral, 
in which only one leaf ever stands at a node, while the one on the 
node next above or below stands part way around the stem, 
the successive leaves falling always into a regularly-ascending 
spiral. Now this space around the stem from one leaf to another is 
a definite fraction of the circumference; in some plants it is }^, 



The Profound Effect on the Structure of Plants 63 









Fio. 16. Diagrams to illustrate the principal systems of leaf-arrangement, as they would 
appear from above if the stems were telescoped to one? piano. The rings are nodes, 
and the* small heavy circles arc leaf bases. Further particulars in the text. 



64 The Living Plant 

as in the Elm and Grasses, in which case one must pass once 
round the stem and cover two spaces to reach a leaf over the 
first (figure 16, C). In others, (e. g., the Sedges), the fraction is 
Va, and a spiral drawn through the bases of the leaves passes 
once round the stem and across three spaces to reach a leaf over the 
first (figure 16, D). In others, (e. g., the Apple) it is 2 / 5 , when the 
spiral must pass twice around the stem and cross five spaces to 
come to a leaf over the first (figure 16, E), an arrangement which 
is, perhaps, the commonest of all. In others the fraction is 3 / 8 
(in Holly and Plantain figure 16, F), or 5 /is, as in cones of White 
Pine, while 8 / 21 , 13 /34, and even some higher fractions are said to 
have been traced in special places where the leaves are greatly 
condensed together in rosettes. And a curious thing is this, that 
while these fractions occur, the various possible intermediate 
ones do not. In these fractions, which primarily express the 
amount of circumference between two successive leaves, the 
numerator also expresses the number of turns that must be made 
around the stem to reach a leaf over the first, while the denomina- 
tor expresses the number of spaces that must be passed over for 
this purpose, and also the number of vertical ranks into which the 
leaves fall. Moreover, these fractions bear to one another a very 
curious relationship, for when they are arranged in a series, viz., 

V2, V3, 2 /5, 3 /8, 5 /13, 8 /21, 13 /34 

it is found that each numerator is the sum of the two numerators 
preceding, and each denominator likewise the sum of its two pre- 
decessors, and moreover each numerator is the same as the de- 
nominator next before the preceding. This curious series, known 
in mathematics as the Fibonacci series, is said to find expression 
in other phenomena of nature, including the arrangement of the 
planets, and is therefore not peculiar to the phyllotaxy of plants. 
The question of present importance, however, is this, what is its 
meaning in connection with leaf-arrangement? Of course one's 
first natural thought is, adaptation, which appears reasonable 
enough with the opposite system and the whorls, and even with 



The Profound Effect on the Structure of Plants 65 

the lower fractions of the spiral system, where one can see the 
advantage of a spacing which may give to the leaves the best 
aggregate exposure to light. But this interpretation meets in- 
creasing difficulties with the higher fractions, and even has trouble 
with the lower when one notices how freely the leaf-blades, the 
very parts which need the exposure to light, are swung by their 
slender petioles into positions of advantageous individual exposure 
in callous disregard of the orderly arrangement in which they start 
from the stem. There is, however, another and very different 
explanation of the systems of phyllotaxy advanced by some in- 
vestigators, viz., that they are wholly determined by the positions 
in which the young leaves originate inside of the growing bud, 
which positions in turn are determined by mechanical principles 
connected with the easiest mode of origin of new swelling parts 
in buds of a certain size and shape. In other words the fractions 
of phyllotaxy are merely an incidental result of mechanical 
conditions present in growing buds, and have only a secondary, 
if any, reference to adaptation. This explanation I believe to be 
substantially correct. It is of course not an explanation of 
phyllotaxy, but merely a transference of the problem into an- 
other field, as most of our explanations are. But I dwell upon the 
subject at this length because phyllotaxy seems to me to offer a 
fairly clear case in which a conspicuous feature of plant structure 
has merely an incidental and not an adaptive origin. 

There is one other feature of leaf and stem structure to which I 
have not yet made any particular reference, and that concerns 
their sizes, which are wonderfully diverse in different plants. 
Leaves are measured in terms of feet in Bananas and Palms, but 
need the assistance of lenses to show them at all in some of the 
kinds that grow in the deserts; they are merely of tissue thinness 
in some kinds of Ferns, but cylindrically-thick and stem-like in 
Aloes and Century Plants. Stems display a thousand feet of 
length in the Rattan Palm, but are invisible supports to tufts 
of leaves in the Houseleek; nearly as thin as a hair in some Ferns, 



66 The Living Plant 

but quite as thick as a house in the larger species of Redwood; 
branched to a spray in a Mango Tree, but an unbranched shaft 
in the Royal Palm. Thus it is evident that leaves and stems ex- 
hibit well-nigh as remarkable a diversity in size as in shape, and 
we must conceive of our generalized or composite leaf and stem 
as well-nigh indefinitely modifiable, possessing, as it were, a 
kind of a super-elasticity in both of these features. As to the 
causes determining size in these parts, that is reserved for dis- 
cussion in the chapter on Protection, where it will be shown that 
the size actually displayed by any leaf or stem represents in the 
main a compromise or truce between the conflicting tendencies 
of the plant to make its leaves larger for photosynthetic advantage 
on the one hand, and smaller for better resistance to hostile ex- 
ternal conditions on the other. 

In this chapter thus far but little has been said concerning the 
root. This is because the consideration of that organ is more 
convenient and natural in the chapter that deals with its function 
of Absorption; and there its description will be found in detail. 
It is enough for our immediate purpose to say that roots, the 
principal organs for the absorption of water and minerals, and 
the third of the primary plant parts, grow out from stems, which 
they closely resemble in structure, having much the same internal 
cellular construction as well as the same long-tapering, freely- 
branching forms. Though not without diversity in form, size, 
and structure, they are yet far less varied in these respects than 
are leaves and stems, and for a sufficient and obvious reason, 
namely, they grow under far more uniform conditions; for life 
in the soil is much the same thing all the world over, however 
varied it may be upon the surface. 

Thus far we have considered only those diversities which leaves 
and stems exhibit while still retaining their typical function of 
photosynthesis. But their remarkable plasticity does not exhaust 
itself here, for these parts can even perform entirely different 
functions, becoming adaptively modified therefor to such a de- 



The Profound Effect on the Structure of Plants 67 

gree that their original nature would hardly be suspected were it 
not for the existence of intermediate stages. And not only that, 
but conversely, substantially all of the structures performing 
remarkable or unusual functions and displaying remarkable forms, 
are simply transformations of the three primary parts, leaf, stem 
and root. This subject of the formation of all the special organs of 
plants out of leaf, stem, and root, (a typical example, by the way, 
of morphological study,) we must now proceed to consider. 

The particular structures performing definite functions in typical 
plants, other than ordinary leaf, stem, and root, are the following : 

Bud coverings, or scales, give needed protection to living buds 
over winter. Adaptively to this function, they are small, con- 
caved, thick, corky, brown, and often resinous, as the large winter 
buds of any common trees will illustrate. 
Bud scales are transformed leaves, usually 
leaf -blades, but in some plants (e. g., the 
Horse Chestnut) are petioles, the blades 
being suppressed, while in others they are 
stipules, as shows very beautifully in the 
Tulip Tree (figure 17.) 

Tendrils, or similar parts, enable slender 
plants to cling to a support and thus mount 
upward towards the light. Adaptively to 
this function they are slender, tough, cy- 
lindrical, or cord-like structures, endowed 

7 FIG. 17. The stipular bud 

with remarkable powers (to be later con- coverings of the Tulip Tree; 

. i * . . ! t , T A i-i'j. \ one-third natural size. 

sidered in the chapter on Irritability), of 

reaching out for a support, taking a firm hold thereon, and sub- 
sequently shortening and toughening their structure (figure 85). 
The best tendrils, like those of the Passion Vine or the Grape, are 
transformed stems, issuing from buds precisely as branches do. 
Others are transformed leaf-blades, as in the curious Lathyrus 
Aphaca (figure 18), or a part thereof, as in Vetches, or Bignonia; 
or are stipules, as in the Wild Smilax, or merely the petiole 




68 



The Living Plant 



which makes a turn around some object, as in the Clematis, or 
a cylindrical part between two portions of blades as in those 
Pitcher plants called Nepenthes (figure 20). In some tropical 

plants, e. g., climbing Aroids, the aerial 
roots clasp horizontally around a support. 
In some others, and notably those having 
the habit of the Ivies, and growing against 
stonework, the tips of the tendrils do not 
twine around a support, but end in discs 
which are firmly appressed to the stones, 
as in the Woodbine, though more com- 
monly the disc-holding structures are aerial 
roots, as the English Ivy illustrates. 

repellingly from some 




FIU. is.-Tendriis trans- 
formed from leaf-blades, kinds of plants as if they might form a 

with stipular foliage, of . , , . . 

Lnthyrus Aphaca; one-half protection against tne attacks oi large 
natural size. plant-eating beasts. They possess a stiff, 

hard, conical structure, and a firm attachment to the skeleton, 

consistent with that use. In some plants they are no more than 

prickles, erupted, so to speak, from the surface, as in the Rose; 

in other cases they are the sharp- 

ened ends of the veins, as in the 

Holly; in others they are the leaf- 

blades, as in the Barberry and 

the Cactus; in others they are 

stipules as in the most spiny 

of the Euphorbias (figure 19), 

though in SOme Other kinds the FIO. 19. The stipular spines of Euphorbia 

spines are the persistent and in- *&*&; o^-haif natural size. 
durated floral branches; in others, such as the Locusts, they are 
transformed branches coining from ordinary axillary buds; in 
some Palms they are roots; and cases are known where they are 
petioles. 
Food Reservoirs store up for later use the food-material made 




The Profound Effect on the Structure of Plants 69 

in the leaves of herbaceous perennial plants, and, adaptively 
to this function, are greatly-swollen, soft-bodied, large-cellular 
structures. They are leaves in the bulb scales of Lilies and Hya- 
cinths, stems in the common Potato (the eyes being axillary 
buds), and roots in the Sweet Potato. 

Insect Traps effect the capture and digestion of insects, and 
thus enable some plants to augment the scanty supply of nitrog- 
enous compounds available where they 
grow. Adaptively thereto these traps have 
highly special forms and accessory features 
contributing to the attraction and capture 
of insects, as will later be noted in a par- 
ticular description of these plants. The 
trap is a pitcher formed by a special cup- 
like-upgrowth of the leaf-blade, as in the 
various Pitcher Plants (figure 20), or else 
a hinged or inrolling blade, as in the Venus 
Fly-trap and Sundew. 

Flower parts contribute in various ways 
to the efficiency of reproduction, as will 
latc appear in a discussion of that subject. 
The parts are transformed leaves, and dis- 
play features adaptive to their functions, 
the green leaf-like sepals which protect 
the other parts while in bud, the brightly- 
colored petals which exhibit the position of 
the flower to the visiting insect, and (though 
with a reservation) the stamens and pistil FlG 20 An insect-trap- 
concerned with the actual pollination. In 




kinds Of flowers the petals are miss- leaf tip in Nepenthes; one- 

third natural size. 

ing, but their function is performed by 

brilliantly-colored leaves close under the flowers, as shown so 

strikingly in the Poinsettia. 

Miscellaneous. There are, furthermore, a great many special 



The Living Plant 



structures with particular functions not belonging in any of the 
definite categories above mentioned. Thus, the bladdery air- 
filled floats which keep the Water Hyacinth resting so lightly 
on the water are petioles; the wing which ensures the carriage 
of the Linden seeds is a leaf-blade (figure 157) ; the indurated hooks 
by which some tropical vines do their climbing are stipules; while 
the reduced or rudimentary leaves which we call bracts often 
also possess functions of a minor sort. 

Substitution foliage. Finally, we must take notice of another curi- 
ous transformation in function and structure found in all parts 
other than the leaf-blade, namely, they may be- 
come transformed into foliage, either in aid of the 
blade, or its replacement. Thus, in some kinds, 
the blade is greatly reduced or missing, and the 
petiole is flattened and thin and acts as the foliage, 
e. g. in the Australian Acacias (figure 21), and 
some kinds of Oxalis. In a good many plants 
the stipules are sufficiently big to render appreci- 
able aid to the leaf-blade. In Lathyrus Aphaca 
(figure 18) they form all of the foliage there is, 
while in the common Bedstraw or Galium, they 
are as large as the leaves and so like them as 
Fl tenedp^tidescrv" commonly to be thought additional leaves helping 
, in i? u? s i f lis ? ge to make up a whorl. In a great many plants, 

(the blades being ^ J x ' 

insignificant), in and especially those found in dry places, the leaves 
Acacia; one-half become very small or are absent, and the function 
natural size. of f o ii age fe performed by the stem, which either 
remains smooth and round, or becomes fluted by the presence of 
vertical green ribs, or becomes flattened in various degrees, all 
three conditions of which are found in the family of Cactuses. In 
some cases the stem is flattened as thin as a leaf, while still dis- 
playing the nodes distinctive of the stem, as in the Muehlenbeckia 
of our greenhouses (figure 22) ; but in other cases no nodes appear, 
knd the stem assumes a form and general aspect so leaf-like that 




The Profound Effect on the Structure of Plants 71 



the botanical teacher has often much ado to convince his students 
that it is anything else, even when he shows them the actual 
leaves, reduced to scaly bracts, out of whose axils the leaf-like 
branches clearly spring. Such is the case with the Butcher's 
Broom of Europe, (figure 23), our common Asparagus, and the 
cultivated Smilax of the florists. Finally there is even a case 





FIG. 22. The Icuf-likc stem, with some small leaves, of Muehlenbeckia; one-half 

natural size. 
FIG. 23. The leaf-like branches of Butcher's Broom; one-half natural size. 

in a tropical Orchid, Taeniophyllum by name, where the roots 
serve as foliage, becoming suitably flattened and otherwise ap- 
propriately constructed. 

We cannot take space to follow any farther this most interest- 
ing subject, but if the reader desires another and much fuller 
discussion thereof, he will find it in the appropriate places in Asa 
Gray's Structural Botany, where it is treated in a manner that in 
my opinion cannot be surpassed. The subject, moreover, is one 
which offers attractive opportunity for concentrated field study 




\\\i i v^Jt < ^ v\ v inVn mi v rep us 



Flo. 24. A collection of specimens, pressed and dried, and arranged to illustrate a 
morphological topic; photographed one-third the original size. 



72 



The Profound Effect on the Structure of Plants 73 

in the discovery, identification, collection and arrangement of the 
various special structures of plants, which can then be preserved 
in some such manner as our picture illustrates (figure 24). 

Thus it is evident that, on the one hand, the three primary 
plant parts, leaf, stem and root, though developed with a 
structure adaptive to the very particular function of photo- 
synthesis or food-making, have in many cases become trans- 
formed into other parts of very different ecological significance 
and structure; while, on the other hand, and correlatively, all 
of the great number of highly specialized parts performing other 
functions can be traced back to an origin morphologically in the 
three primary plant parts. This interlocking relationship of 
morphological origin with ecological meaning, of morphology 
with ecology, can perhaps be made clearer by use of a diagram 
such as is given herewith (figure 25). 

Although I ought now to end this long chapter, I will continue 
far enough to answer two questions which I am sure have arisen 
in the mind of the reader. Thus, he will surely be wondering 
why it is that some plants make their tendrils, for instance, from 
leaf-blades, others from petioles, others from stipules, others from 
steiAs, and others even from roots. The most reasonable answer 
appears to be this, that when a plant, owing to a change of habit 
forced on it by a change of environment, develops a need for a new 
organ, that organ is made by a transformation of the part which 
happens to be most available for the purpose, often some part 
which the change of habit has happened to set free from its 
former use; and sometimes that most available part will be one 
thing and sometimes another. In the second place the reader 
will wonder why some plants should abandon their leaf-blades 
as foliage, and then proceed to replace them by petioles, stipules, 
stems, or even roots, which are for the purpose converted physi- 
ologically and structurally into leaves. In answer it may be said 
that the abandonment of the leaf-blade, as will be shown in the 
chapter on Protection, usually accompanies exposure to very dry 



74 



The Living Plant 



Leaf-blades 



Petioles 



Stipules 



Stems 



Roots 




Flower parts 



Foliage 



Insect traps 



Bud covers 



Tendrils 



Spines 



Miscellane- 
ous 



Support to 
foliage 



Storage 



Absorption 



FIG. 25. Diagram to illustrate the interrelations of morphological origins with ecological 
uses in the parts of the higher plants. 

climate, in which case the function of foliage is taken over by 
some other part, usually the stem. Now it is conceivable that 
when, by another change of habit, the plant finds itself in need 
of a much larger spread of chlorophyll surface, this may be more 
easily obtained by further enlarging and flattening the already 



The Profound Effect on the Structure of Plants 75 

leaf -like stem than by re-developing the lost leaves. It is probable 
that some peculiarity of this kind in the past history of the plant 
will explain in each case such curious features, the course of devel- 
opment being always that which offers the least resistance at the 
moment. 

The reader will now be prepared, I think, to admit that of all 
the influences concerned in the determination of plant form, 
indeed in making plants what they -are, the most important by 
far is the physiological process of food-making, or photosynthesis, 
and that the feature of this process having the most profound 
effect is the need for exposure to light. 




CHAPTER IV 

THE KINDS OF WORK THAT ARE DONE BY PLANTS, AND 
THE SOURCE OF THEIR POWER TO DO IT 

Respiration 

HEN first I had written this chapter, and made it the 
best that I could, it assumed that the fact of plant 
work was already well-known to the reader. A later 
experience, however, made me see very clearly that 
most people do not know that plants work at all. Accordingly 
I shall make it my first endeavor to show beyond question that 
plants do work; then we can pass with better understanding to 
the study of the very remarkable source from which they derive 
their power to do it. 

The principal reason why the majority of people do not as- 
sociate with plants the idea of work is found in the slowness of 
most plant actions. Our conception of work is almost entirely 
subjective, and because plants are placid of mien, and do not hurry 
and fret and strain, we think they are doing no work. When the 
Master said of the Lilies, that they toil not neither do they spin, 
his words expressed the popular fancy but not the physical fact. 
Work is none the less real because it is slow, and the matter of 
slowness is entirely relative and subjective. Even the very swift- 
est actions performed by any of us must seem slowness person- 
ified to the lightning, or to a dynamite charge which can finish 
its work before you can think, or to the forces of collision which 
reduce a railway train to a heap of tangled scraps within the 
space of an instant. Probably the lightning, the dynamite, or 
the collision forces, if interviewed on the subject, would say that 

7 6 



The Kinds of Work That Are Done by Plants 77 

mankind does not work. But if plant actions could be magnified 
immensely in speed they would impress one very differently in 
this particular. For then the observer would see the tip of every 
growing plant-structure nodding and moving energetically about, 
so that a meadow, a copse, or a forest would seem all of a vigor- 
ous tremble as if straining at some hidden leash : he would see the 
buds of some flowers open and close with a straining yawn or 
a sudden snap, and others burst into bloom like a rocket when it 
breaks to a spray of mani-colored lights: roots in their efforts 
to penetrate the earth turning and twisting like angleworms im- 
paled on the fisherman's hook: seedlings in their struggle to break 
through the ground heaving and straining at their burden of 
superincumbent soil, like a powerful man at some load which 
has fallen upon him: seed pods pushing into the earth on a twist- 
ing or hard-thrust stalk : tendrils swooping in curves through the 
air, gripping the first thing they meet, and jerking their plants 
towards the support. As matter of fact, there does exist a way 
in which we can readily behold these actions thus magnified, 
for if the structure in question be photographed at regular inter- 
vals, say of fifteen minutes to half an hour, and then these photo- 
graphs are run at high speed through a moving-picture machine, 
the thing is done. Such studies have actually been made in the 
case of twisting roots, moving fruits, and opening flowers; and all 
of those who have seen them agree in the impression of vigorous 
work thus presented. 

Furthermore, if we could magnify in like manner the interior 
parts of the plant we should witness as remarkable actions pro- 
ceeding with equivalent vigor. In some plants the living proto- 
plasm would be seen flowing in thick turbid streams round and 
round within the encasing cell- wall; in certain cells those re- 
markable structures called chromosomes would be seen perform- 
ing their curious manoeuvres, arranging themselves into groups, 
collecting in pairs, passing backward and forward in a manner 
suggestive of the measures of the dancers in a- quadrille; else- 



78 The Living Plant 

where new cells would be seen in process of birth, and engaged in 
forcing the older apart to make room for themselves; while minor 
actions without number, mechanical, physical, and chemical, 
would appear in vigorous progress in various parts of the organ- 
ism. Truly if one could see these actions under the conditions 
here imagined, he would have no trouble at all in connecting 
with plants the idea of real work. 

We are not, however, dependent solely on imagination, or 
the moving-picture machine, for a conception of the reality of 
plant work. The rapid closing of the leaf of a Venus Fly-trap 
upon a captured insect, or the sudden collapse of the Sensitive 
Plant when touched, suggest some such idea. Everybody has 
noticed that the great granite curbstones along streets where 
shade trees are grown, become heaved from the regular lines in 
which they are laid, while the pavements themselves are often- 
times thrown into irregular swells; this is all brought about by 
the growth of the roots of the trees, which thus exhibit a work as 
real as that of a jack-screw or derrick. If the reader has not al- 
ready observed these phenomena, let him do so when next he 
walks through a shaded street. In a similar manner young roots, 
insinuated between the stones of buildings, tombs, or walls, 
force the masonry apart in their growth, and finally accomplish 
the destruction of the edifice. Occasionally asphalt pavements 
are burst upwards by the growth of some kinds of plants, including 
even soft-bodied Fungi, as the accompanying photograph well 
proves (figure 26). And the technical literature of plant physi- 
ology tells of the thousands of pounds pressure exerted by large 
gourds, like Squash, when suitably harnessed to recording machin- 
ery. And, finally, experiment proves that every operation of 
plant life, even the least of them all, involves some movement, 
and therefore real work; so that animals and plants are working, 
and often right hard from the physical point of view, when they 
merely are keeping alive, a conclusion from which the reader 
is welcome to draw any comfort that he can. 



The Kinds of Work That Are Done by Plants 79 

At this point, perhaps, some one will rise and declare I am wrong 
in my statement that work is as real when slow as when swift. 
But note that I say as real, not as hard. When a weight of a ton 
is lifted a foot, no matter by what means, the work is the same 
whether done in a day or a minute, although it is over a thousand 
times harder to do, (to be exact, the power required, is 1440 times 
greater) in the latter case than the former. But the fact of im- 




FIG. 26. An asphalt pavement burst upward by the growth <>i soft-bodied mushrooms, 
whose conical heads are visible over the wreckage. 

mediate importance is this, that the work is as real in one case 
as the other. 

We come now to the bond of connection between this matter 
of plant work and the principal theme of this chapter, viz., it 
is a fact of physics, which the reader must long since have learned, 
that every bit of work of every kind done anywhere whatsoever 
in nature, whether in a plant, or an engine, or the skies, or the 
thinking brain of a man, requires for its accomplishment the 
presence and expenditure of energy, which is the source of all 
power. The reader, of course, knows what energy is, the en- 
tity in Nature, and the only one, that produces motion by which 



8o The Living Plant 

work is accomplished. Energy is most familiar as heat or elec- 
tricity, though manifest also in light and in chemical reactions. 
Without energy there is no motion, no power, no work; and with- 
out it a plant or an animal stops as dead as an engine when no fire 
burns under its boiler. Plant work, therefore, requires and im- 
plies a supply of energy. And with this conclusion it will be well 
to gather the foregoing matters into a generalization, another 
of our botanical verities; all plants, like all animals, are inces- 
santly at work while alive, as truly as any moving machine, not only in 
the performance of their active and visible movements, but also in the 
bare maintenance of their existence; and this work requires a pro- 
portional supply of energy. 

It is now our business to find the source of the energy by which 
plants do their work. We know the source of the energy in the 
work of the engine just mentioned; it is the heat released from 
the burning of coal in a grate. But what is the source of the energy 
in the work of the plant, which has neither grates, nor boilers, 
nor flaming of fuel? 

When the student of science is faced by a problem like this, 
his first resource is to look around for suggestions from some 
analogous process. In this instance he would turn naturally 
to animals, and his earlier studies on the physiology of man would 
have taught him that the power of animals to do work is connected 
in some way with their respiration, that process in which they 
give forth the gases carbon dioxide and water vapor to the air, 
while absorbing the gas oxygen into their bodies. How inti- 
mately this process is connected with work is easily realized 
when we recall the familiar fact that respiration increases in pro- 
portion as work becomes harder. Is it possible, then, that 
plants also respire? That is, do plants in their work release car- 
bon dioxide, and absorb oxygen? Obviously this matter is de- 
terminable by experiment, and the following is a very good 
method. In a bottle arranged as shown by the picture (figure 
27), we place some plant parts which are actively working with- 



The Kinds of Work That Are Done by Plants 81 



out the complications introduced by photosynthesis (e. g., ger- 
minating seeds, such as Oats), then close the bottle air-tight by 
means of the stoppers and clamp provided for the purpose, and 
stand it for some hours in a warm 
and dark place where growth can 
take place. Obviously, any carbon 
dioxide released by the seeds must 
collect in the bottle, where its pres- 
ence may be detected by its well- 
known property of turning clear lime- 
water milky. If, accordingly, clear 
limewater is poured into the tall vessel 
into which the delivery tube leads, 
the clamp is loosened, and water is 
poured down the thistle tube, then 
the gas will be forced from the bottle 
and sent bubbling up through the 
limewater. The result is always de- 
cisive. The limewater turns white- 
milky proving the presence of car- 
bon dioxide in abundance. And if 
a bright person should here rise to 
remark that the carbon dioxide al- 
ways present in air is sufficient to ex- FIG. 27. A Respiroscope, or ar- 

... IA -A A *x rangcment for demonstrating that 

plain the result, It IS easy tO prove It plants respire. Its operation is 

is not; for, if an equal quantity of air explaincd in thc text ' 
be forced from an empty bottle through limewater no milkiness 
appears. Arid if, in the bottle, we place buds, or roots, or color- 
less plants like Mushrooms, or even green leaves (in the dark), the 
result is always the same. Furthermore, it is also the same whether 
the working parts are kept in the light or the dark, and it is still 
the same, as the reader may be confounded to learn, even with 
green leaves when kept in the light, though here the process is 
obscured by the absorption of that gas in photosynthesis, as can 




82 



The Living Plant 



be proven by experiments, too elaborate, however, for description 
at this place. Furthermore, as we may conveniently note here, 
all of these same working parts are simultaneously releasing water 
as well. It is therefore true, as a general principle, that all working 

parts of all plants are 
giving off carbon dioxide 
as well as water, pre- 
cisely as animals are do- 
ing. 

But do plants exhibit 
the other phenomenon 
of animal respiration, 
absorption of oxygen? 
It is very easy to prove 
that plants must have 
oxygen in order to live 
and work, precisely as 
animals must ; for if two 
sots of the same seeds are 
placed in two similar 
closed chambers, and 
then the oxygen is re- 
moved from one chamber 
by a chemical absorbent 
while it is left untouched 
T , ou , * i i. i 11 in the other, the seeds in 

ri(j. 28. Iwo similar tube-chambers in which were J 

placed similar sets of germinating oats kept wet the OXygeilleSS chamber 
and in place by wads of moss, and treated pre- 

ri.s.-ly alike except that those on the right were de- Will not germinate at all 
privcd of oxygen. i MI j i i 

and will soon die, while in 

the other they will grow normally for a considerable time (figure 
28). Furthermore, if the air of a closed chamber in which seeds 
have been growing for some days be subjected to chemical 
analysis, it is found that most of the oxygen has disappeared 
from the chamber, and must therefore have been absorbed by 




The Kinds of Work That Are Done by Plants 83 

the seeds. And the same thing is true no matter what structures 
we place in the chamber (saving only an apparent exception, 
soon to be noted, in the case of lighted green leaves), and no 
matter whether the chamber is exposed to the light or kept in 
the dark. It is evident, therefore, that all parts of working, 
(and that is to say, of living) plants, absorb oxygen and release 
carbon dioxide precisely as animals do. 

There is no one, I think, who can grasp fully the bearings of a 
complicated subject after only a single presentation, no matter 
how clear this may be. It is therefore quite likely that some reader 
ere this has experienced a feeling of dazement, and been led to 
exclaim, along with the much-puzzled German, "Jemand ist 
verriickt, aber wer?"; and he may even incline to imagine that 
I am the "wer." For have not I shown, in an earlier elaborate 
chapter, that plants absorb carbon dioxide and release oxygen, 
while now I have proven by evidence quite as conclusive that 
they do exactly the opposite? But there is, nevertheless, no in- 
consistency. For the reader will recall that it is only the green 
tissues which absorb carbon dioxide and release oxygen, and then 
only in light, and then only from the tiny little chlorophyll grains 
embedded inside of the protoplasm. There should therefore be 
no trouble in understanding how the protoplasm in which those 
grains are embedded, like all other living parts of the plant, can 
be respiring, while the chlorophyll grains alone are engaged in the 
photosynthetic process. The case of the chlorophyll grains, 
however, is not so simple as my statement implies, because, 
since they are living protoplasm, there is every reason to think 
that they also respire even in light, and that in them, and in 
them alone, the two processes go on together. If, now, photo- 
synthesis and respiration, with their exactly opposite gas ex- 
changes, proceed together in leaves, why do they not neutralize 
one another's results? The answer is easy. Experiment shows 
that on the average the photosynthesis in green leaves in the 
light is over twelve times as active as respiration (and it may rise 



The Living Plant 



very much higher), a preponderance that is obviously so great 
as to over-balance not only the respiration of the leaves, but of all 
the remainder of the plant besides, and not for daytime alone, 
but also for night. Therefore, day and night together, the green 
plant absorbs much more carbon dioxide than it releases and re- 
leases much more oxygen than it absorbs. It vitiates the air by 
its respiration, but in the long run purifies it still more by its 
photosynthesis. 

Before leaving this part of our subject, we should look a little 
more closely into the relations of the two processes within the 






FIG. 29. Diagrammatic sections across leaves, to illustrate the movements of gases in 
and out of the same during, a, light, c, darkness, and b, the balance period between. 
The squares are carbon dioxide, the triangles are oxygen, and the arrows show the 
direction of movement. 

lighted green leaf, a subject diagrammatically illustrated by the 
accompanying figures (figure 29). At night all of the carbon 
dioxide given off by the respiration of the living cells into the air 
passages, makes its way along these and through the stomata 
to the atmosphere outside, (figure 29, c). In the daytime any 
carbon dioxide given off by the respiration of the protoplasm is 
absorbed by the chlorophyll grains in the same cells, but as this 
supply is wholly insufficient, a constant stream of that gas passes 
in from the atmosphere through the stomata and along the pas- 
sages to the different cells, where it is absorbed by the chlorophyll 
grains; simultaneously a part of the oxygen given off by the 
chlorophyll grains is absorbed by the protoplasm of the same cells 
for their respiration, while the very large surplus is sent into the 



The Kinds of Work That Are Done by Plants 85 

air passages and along them and through the stomata to the at- 
mosphere; and the reader should thus visualize these matters in his 
imagination (figure 29, a). But here comes an interesting point. 
Since photosynthesis is dependent upon light while respiration is 
not, there must evidently exist a certain intensity of light at which 
the two processes in a leaf exactly balance. At such times the 
processes use one another's gases, and there is no movement of 
carbon dioxide or of oxygen either into or out of the leaf (figure 
29, 6). Such a balance period must occur every day just after sun- 
rise and before sunset, and on some very dark days it probably 
lasts for considerable periods. It is of course by virtue of approx- 
imation to such a balance that some kinds of plants such as Ferns, 
if not given too much light, can thrive so well for long periods 
of time in tightly-closed cases, or masses of red-berried vines 
(Partridge-berry) can exist all winter in little closed globes ori 
dining-room tables. 

We may now express the important facts of the past few pages 
in another of our botanical verities, to this effect, that plants, 
like animals, respire, and in identical manner, absorbing oxygen 
and releasing carbon dioxide, throughout all of their living parts. 

In the preceding paragraph I have said that the gases enter 
through stomata and pass along air passages, but I have given 
no hint of the forces which impel them. This matter will be taken 
up fully in the chapter on Absorption, where it will be shown 
that the gases move along diffusively under action of forces 
internal to themselves. We need only note here that plants have 
no system at all for absorbing and expelling large masses of air 
as animals do by the use of their chest-muscles and lungs, an 
operation that is always called breathing. Accordingly, the matter 
can be stated in this way, that plants respire, but do not breathe. 

It will be well, at this point, to turn aside for a moment from 
our main subject to consider some phases of plant respiration 
which have economic importance. The first is concerned with 
aeration of soils. Roots, like all other living parts, must respire 



86 



The Living Plant 



in order to grow, and, with the exception of a few which possess 
long air passages connecting with the leaves, they take the in- 
dispensable oxygen from air in the soil, by a method to be later 
explained. A soil in the best condition for the respiration of roots 
has the structure represented, under large magnification, in the 
accompanying picture (figure 30). Soil is formed of particles 




FIG. 30. A generalized drawing of a section, highly magnified, through a well-conditioned 
soil and a fragment of root. The soil particles are dotted, the water is concentrically- 
lined, the air spaces are left blank; into the soil project the root-hairs from the root 
on the left. (Improved from a picture in Sachs' Lectures.) 

of rock, irregular in size and form. Around these particles and 
in the angles between them is water, held in the capillary state, 
while bubbles of air exist in the larger of the spaces among the soil 
particles. When more water is added, then the air, being lighter, 
is driven upwards and comes bubbling out of the ground; but 
it returns again as the surplus water drains or evaporates away. 
It is from this air in the soil that roots take their oxygen, and if 
the air is kept out of the soil by excess of water, then the roots are 
suffocated and die, precisely as air-breathing animals do when they 



The Kinds of Work That Are Done by Plants 87 

are kept under water. Roots, in fact, drown as truly and in ex- 
actly the same physiological way as do animals, and with only 
this difference, that roots can stand immersion for hours or days, 
while animals can endure it only for minutes. This explains 
the need for drainage of wet soils; it is not that these have too 
much water, but too little air. It explains also why the soil of 
flower pots needs to be carefully drained, and the cause of the 
failure of so many persons in the care of their house plants, which 
most people keep too constantly wet. The very best treatment 
for most potted plants is to give to the soil an occasional soaking, 
and allow it to dry out pretty well in between times; the roots do 
not mind the absence of air for some of the time if they can have a 
sufficiency at other times. Moreover this method of watering has 
another great advantage over that of adding a little water more 
frequently, in the far greater effectiveness with which it drives 
out the foul air and ensures a fresh supply. 

Another economic phase of respiration is involved in the 
popular belief that it is unhealthful to keep house plants in sleep- 
ing rooms. It will now be plain to the reader that this belief is 
correct. But in fact the danger is slight. The amount of carbon 
dioxide given off in respiration by a square meter of leaf is only 
about the three-hundredth part of that given off in the same time 
by a person, and although buds and roots respire more actively, 
it is likely that a whole window-full of plants does not give off 
one fiftieth of the amount that one person does. Or, it has been 
stated thus, that all of the plants which could be crowded into 
the windows of any ordinary sleeping room give off less carbon 
dioxide to this air than would a tiny light kept burning over night; 
and nobody would consider this quantity injurious, especially 
if the room were ventilated as it should be. Indeed, were the 
respiration of the plants in a room not negligibly small, it would 
obviously be unsafe for any person to camp out in a forest in 
summer! 

We must now come back to the more technical aspects of res- 



88 The Living Plant 

piration, and examine more closely the chemical and physical 
aspects thereof. Since the plant, in this process, absorbs oxygen 
only, but releases carbon dioxide, a question is raised as to the 
source of the carbon. This must come, of course, from some of 
the innumerable carbon-holding compounds inside of the plant, 
but, for our present purpose it does not much matter from which, 
since they all are derived by transformation from the basal 
grape sugar manufactured in the leaves. This grape sugar, ac- 
cordingly, is the ultimate, even though not the immediate source 
of the respiratory carbon. Therefore we can state the end prod- 
ucts of respiration in this wise : 

In respiration C 6 H 12 6 and 2 form C0 2 and H 2 O 
grape sugar oxygen carbon dioxide water 

This general statement can be given a definite chemical form 
by making the two sides sum up alike, which requires these pro- 
portions: 

CoH 12 6 + 6 2 = 6 C0 2 + 6 HO 

Now although this equation is rarely if ever actually realized 
in any particular case, (respiration being never so simple, but a 
process highly complicated in its details), it does represent the 
facts as to the ultimate materials and products, the two extremes 
of the process; and accordingly we may place it in our series of 
conventional constants as the respiratory equation. And its 
relations to the photosynthetic equation will not escape the notice 
of the observant reader. The two are the exact reciprocals of 
one another, which fact is one of the most consequential in all 
nature, as will presently appear. 

And now we come to a matter which I wish to impress, the 
strongest I can, on the mind of the reader. The phenomena we 
have thus far considered, including the one which stands for 
most people as the very embodiment of the process, viz., the 
remarkable exchange of the gases, are by no means the ones of 
greatest importance in respiration, but are secondary and in- 
cidental to the central and crucial object of the process, which 



The Kinds of Work That Are Done by Plants 89 

is this, the release of energy. This release takes place in a single 
perfectly definite way, namely, as the result of the invariable 
physical fact of Nature that at the instant carbon unites chemi- 
cally with oxygen, it matters not in what place or under what 
circumstances, energy is released. It is for the release of this 
energy that the process of respiration exists; and the plant no more 
respires for the purpose of absorbing oxygen and releasing carbon 
dioxide than we kindle a fire in the grate in order to make oxygen 
rush into the furnace or carbon dioxide pour out of the chimney. 
The object of respiration and of building the fire (i. e., of com- 
bustion), are one and the same, namely, to secure that energy 
which is always released at the moment of chemical union of 
carbon with oxygen. Respiration and combustion are strictly 
homologous terms, applying to phenomena which are also homol- 
ogous. In the combustion of coal, which is carbon, in a grate, 
the energy is released chiefly as heat (with some light) ; and by 
causing that release to occur underneath a suitable arrangement 
of boilers, pistons and wheels, the energy can be made to produce 
motion and thus do work, as every steam engine is a visible wit- 
ness. In the explosion (which is merely a rapid combustion), of 
gasolene and oxygen inside the cylinder of an automobile engine, 
we have exactly the same thing with a very much simpler machin- 
ery. In respiration within the cells of an animal or a plant, the 
machinery is simpler still, but the principle remains the same; 
the energy is released at the moment of oxidation under such 
conditions that it acts on the simple protoplasmic machinery 
provided by the plant in a way to secure transformation into 
motion and work. The source of the energy of the work done 
by the engine and plant is identically the same; it is only the in- 
termediate machinery which is different. The nature of this 
machinery, it is true, is not at all understood in the plant, but we 
know that something of the kind must exist. The machinery 
must also differ somewhat for the different kinds of work that 
plants and animals do; but in all cases it is driven by one and the 



go The Living Plant 

same power, which depends on the energy released by the oxida- 
tion of carbonaceous food. And it may interest the reader hav- 
ing a turn for figures to know that the energy released by the 
respiration of sugar is just about half of that released by the com- 
bustion of an equal weight of the best coal. 

These matters though clear on reflection, are hard to grasp in a 
first presentation; and I suggest that we rest a little by consider- 
ing an incidental matter of interest. In the foregoing paragraph 
I implied that the energy of respiration is not released as heat, 
and thus differs from combustion. But the implication is not 
strictly correct, as is easily proven. If one takes two handfuls 
of seeds, soaks them, and starts them growing and therefore 
respiring, kills one set by hot water, places them both in good 
non-conducting chambers provided with thermometers, and leaves 
them some hours, he will notice a remarkable result. The ther- 
mometer in the living and respiring seeds will soon read several 
degrees above that in the others, which are obviously similar in 
all ways except that they cannot respire. And further experi- 
ment shows that this release of heat by these respiring seeds is rep- 
resentative of all respiring parts, and that the release of heat is a 
constant accompaniment of respiration. Although usually small 
in amount this heat sometimes becomes readily recognizable. 
Thus the rapidly-opening flowers of Aroids (our Jack-in-the-Pulpit 
and its relatives) often show by the thermometer a temperature 
several degrees above that of the air; some alpine flowers can melt 
their way up, by aid of this heat, through the snow; grain germi- 
nating or fermenting in large masses becomes often noticeably 
warm; the warmth of hot beds derived from fermenting manures 
has the same origin, though here the respiration is that of bac- 
teria or molds; and various cases of spontaneous combustion, 
where correctly reported, must have the same origin. It does not 
appear that this heat, in plants at least, secures any physiologi- 
cal advantage but is rather an incidental result of the physical 
forces at work, very much as incandescent electric lamps made 



The Kinds of Work That Are Done by Plants 91 

primarily to give only light incidentally give much heat as well. 
But it is this very same heat developed and kept in regulation 
which is the basis of the uniform warmth of the animal body. 
A few pages earlier it was shown that the carbon in the carbon 
dioxide released in respiration comes from inside the plant. This 
being so, respiration ought always to entail a loss of weight in 




FIG. 31. Plants of Buckwheat grown from the same number and weight of seed in light 
and darkness respectively. The plants arc in porous saucers supplied with water and 
minerals from below. 

respiring plants or animals; which in fact is found by experiment 
to be true. The loss must be compensated by new supplies of 
food, else the phenomena of starvation, including emaciation, 
ensue. The emaciation of a starved animal, indeed, is due much 
more to the loss of substance through respiration than through 
the ordinary excretions. In plants, however, it often happens 
that those which have lost much weight by respiration without 
opportunity to make it up by photosynthesis, look larger than 



92 The Living Plant 

others which have done the normal photosynthetic work, the ex- 
tra bulk being nothing but water. Thus, the two sets of plants 
in the accompanying picture (figure 31), were started by the water- 
culture method, (later to be explained), from two sets of seeds of 
exactly the same weight. But one set (that on the left) was grown 
in the light and was able, therefore, to make up its loss by photo- 
synthesis, while the other was grown in the dark and could not. 
Yet the latter, owing to the habit of plants to spindle out greatly 
in length in darkness, actually look larger than the former. 
When, however, I weighed these two sets after all of the water 
has been dried out, leaving only dry substance behind, the smaller 
lighted plants weighed a good deal more than the larger ones 
from the dark. It can always be accepted as true that respiration 
entails loss of weight through the loss of carbon from the plant. 

We can now gather up the facts set forth in the preceding pages 
in another of our generalizations, or verities, the energy indis- 
pensable to the work of plants is principally provided by the oxida- 
tion of carbonaceous food, and this is the essential feature of respira- 
tion. 

In the statement of the foregoing verity the reader will notice 
that I have used the word "principally/' thus implying that 
some other source of energy is available. In fact, while respiration 
supplies by far the larger part of the energy used by organisms, 
and especially by animals, they do derive some small part from 
other sources, notably the heat of the surroundings. But this 
part of the subject will all be elucidated later in this book. 

We are now face to face with a question of a very fundamental 
sort, namely, what is the source of that energy which is thus 
released from food in respiration? For everybody knows that 
energy is not created upon the spot, but originates only by 
transformation of pre-existing energy. In all science there is no 
principle better established, or more important, than that of the 
conservation of energy and matter, which teaches that the sum 
total of both energy and matter in nature is constant, and that 



The Kinds of Work That Are Done by Plants 93 

none of either is ever created anew or obliterated, though they 
may change their forms multifariously. Where, then, and in 
what form was the energy in food before it was released by respir- 
ation? The answer is easy, though its comprehension is not. 
It was where the energy was in the coal before it was released 
as heat in combustion: where the energy was in the storage bat- 
tery before it turned the wheels of the electric automobile : where 
the energy was in the coiled spring or the wound-up weight of 
the clock before it turned the wheels to move the hands: where 
the energy was in the full millpond before it drove the looms of the 
water-power mill: where the energy was in the gunpowder before 
it started the flying bullet. The fact of the matter is this, that 
energy exists in Nature in two different forms, not only in the 
familiar active or kinetic form which produces motion and does 
work, but also in a resting, latent, or potential form, when its 
power to produce motion is held in suspension. Whenever, in 
Nature, kinetic energy is exerted to force apart bodies whose 
attractions, whether through gravitation, magnetism, cohesion, 
or chemical affinity, tend to bring them together, the energy goes 
into the potential form for so long as those bodies are kept apart, 
and it becomes again manifest in kinetic form when the bodies 
are allowed to re-unite. All unsatisfied attractions in Nature are 
latent energy. When a small boy draws back the powerful elastic 
of his favorite sling-shot, he is exerting kinetic energy against 
the tension of the elastic; while he holds the elastic stretched to 
take aim, that energy is latent as energy of tension; and when he 
lets go of the string the energy becomes kinetic again as it drives 
the stone in delightful swiftness of flight. So, kinetic energy can 
raise a weight, go into the latent form as energy of position while 
it is suspended, and come out again in kinetic form, as it does 
when it turns the wheels of an old-fashioned clock. Kinetic 
energy can charge a storage battery, become latent for a time, 
and come out once more as kinetic energy driving an electric 
automobile. The storage battery, indeed, is typical of all cases 



94 The Living Plant 

where energy is potential in the form of unsatisfied chemical 
affinity. The electric current forces apart the tightly-cohering 
atoms of certain very stable chemical compounds; but these atoms 
nevertheless retain all their old attraction for one another, and 
it is in the form of this unsatisfied attraction that the energy 
is latent; and this energy is given out again in kinetic form at 
the moment when the atoms are allowed once more to unite. 
Now the very same thing is true of carbon dioxide, which is a 
very stable substance of tightly-cohering atoms. To force apart 
carbon dioxide into its constituents requires kinetic energy, 
which then remains in the latent form, as energy of unsatisfied 
chemical affinity, so long as the carbon and oxygen are held apart, 
but becomes kinetic again when the carbon and oxygen are al- 
lowed to reunite to carbon dioxide. Does the reader see the ap- 
plication? Surely he must. The kinetic energy of the sunlight 
splits apart carbon dioxide in the green leaf, the oxygen going 
out to the air and the carbon combining with the elements of 
water into grape sugar; so long as this carbon and oxygen are kept 
apart, that energy is latent in the form of unsatisfied chemical 
affinity; and when the carbon of the sugar (or of any other sub- 
stance into which the sugar is transformed) is allowed to unite 
with the oxygen of the air, as it is in the process of respiration, 
then kinetic energy is again given out and can be used for the work 
of the plant. Such is the source of the energy of respiration, 
it is energy released from the latent state in food, where it was 
placed (or "stored ") by the kinetic energy of the sunlight. Food, 
therefore, is a storage battery charged by the sun, and discharged 
by respiration. 

The principal function of food must now be quite plain. As a 
storage battery it has advantage over any that man has yet 
made in the fact that it can be reduced to very small fragments, 
or even to solution (by digestion), and thus transported to all 
parts of plants and throughout the bodies of animals. Then, at 
the spot where work needs to be done, just at the right instant, 



The Kinds of Work That Are Done by Plants 95 

under the suitable machinery, the carbon of the food is allowed 
to unite with oxygen, and the energy is released to do the need- 
ful work. And that is the way in which plants and animals ac- 
complish their work; and the power to do this, to absorb stored 
energy, transfer it to all of their parts, hold it ready for use, and 
release it when needed, is the most distinctive feature of living 
beings. 

The reason is now evident also for the reciprocal character 
of the photosynthetic and respiratory equations. In photosyn- 
thesis carbon dioxide and water are made into sugar and oxygen 
with storage of energy; the sugar is transported by plants or by 
animals to places of need, undergoing chemical changes on the 
way but ever retaining the store of unsatisfied carbon; then in 
respiration oxygen is allowed to come into chemical contact with 
the sugar, and the two are changed back to carbon dioxide and 
water with release of energy. It is because substances exist which 
thus permit of such storage and transportation of energy that 
organisms as we know them are possible. 

It may aid still more to a clear understanding of these two most 
fundamental and important of all physiological processes if we 
set their chief features in contrast in form of a table; 

Photosynthesis Respiration 

Occurs only in plants Occurs equally in plants and animals 

Occurs only in chlorophyll grains Occurs in all living protoplasm 

Occurs only in light Occurs equally in light and darkness 

Manufactures food Destroys food 

Increases weight Lessens weight 

Absorbs carbon dioxide Releases carbon dioxide 

Releases oxygen Absorbs oxygen 

Forms CcHiaOe from C0 2 and H 2 Reduces C 6 Hi 2 O fl to C0 2 and H 2 

Stores energy Releases energy 

We can now gather up these latter facts in another of our 
verities thus, the energy released in respiration ivas previously 
latent in the unsatisfied affinity of the carbon in tiie food for the 



96 The Living Plant 

oxygen outside, those two elements having originally been separated 
by the kinetic energy of the sunlight in photosynthesis and kept 
separate through all the subsequent transformations and trans- 
portations of the food through the bodies of plants and animals; the 
original source of respiratory energy is therefore the sunlight, and 
food is primarily a storage battery, charged by the sun in green 
leaves and discharged by respiration at the places of need. 

It will doubtless ere this have occurred to some philosophic 
reader to ask whether carbon dioxide and water are the sole 
substances by which organisms can thus store and transport 
energy, and whether, accordingly, life is dependent solely upon 
them. There is, however, no chemical reason why organisms 
might not use in the same way any other decomposable and 
oxidizable substances, and indeed even in our common plants some 
small quantity of energy is no doubt derived from the oxidation of 
other elements, while certain Bacteria exist which can use the 
energy derived from the oxidation of sulphur compounds. Plants 
probably use carbon in photosynthesis and respiration chiefly 
because its chemical transformations, which are very susceptible 
to temperature, happen to be easily under control at the temper- 
atures now prevailing on the earth's surface. Under markedly 
higher or lower temperatures carbon would be unavailable for this 
purpose, but it is conceivable that life might still exist by the 
similar use of other substances whose combinations would be 
under control at those temperatures. It is only a step farther to 
assume that life might even exist in this way in the flames of a 
nebula, or the awful cold of interplanetary space, and hence 
that its origin may be contemporaneous not only with the origin 
of the earth, but even with the origin of matter itself. It is not 
at all likely that life is something which results incidentally from 
the properties of carbon; it is far more probable that it is some- 
thing which uses the properties of carbon as the most convenient 
tools for its own ends. This is a phase of the super-vitalism of 
which I have spoken in the first chapter. 



The Kinds of Work That Are Done by Plants 97 



This chapter has already attained to a length so great that I 
wish it were possible to end it right here. But certain additional 
matters are connected with respiration so closely, and are be- 
sides in themselves so important, that we must really keep on to 
include them, though perhaps the reader will find it best to defer 
a reading thereof for another occasion. These matters are fer- 
mentation, decay, and disease. 

Fermentation is a phenomenon familiar to all, and best known, 
perhaps, in the "working" of preserves, which become "strong" 
i. e. alcoholic, while giving off tiny 
bubbles of gas. The most typical 
kind of fermentation is that caused 
by Yeast. Yeast, I venture to 
remind the reader, is a very tiny 
non-green plant which lives as a 
saprophyte in sweet liquids. Mag- 
nified to a high degree by the mi- 
croscope it looks much like our 
picture (figure 32), though whiter. FIG 32 _ Yeast plantS|Cach a singlc cell 

A Yeast plant is a Single OVoid which buds out from a parent cell; very 
. highly magnified. 

cell which buds out into others, 

and these into others, in loose chains which fall easily apart, 
and so on, as long as the food supply lasts. And that is all, 
except that when the liquid dries up, the cells produce very 
thick-walled spores which float around in the air with the dust, 
to start once more when they happen to fall into another sweet 
liquid. It is by the growth of these cells that a sweet liquid is 
"fermented" with a formation of alcohol and carbon dioxide. 
This can be demonstrated very easily and clearly to the eye by 
an interesting experiment. If one puts together in a glass flask 
a solution of sugar and a cake of compressed (not dried) yeast, 
and stands it in a warmish place, then within a very few 
minutes tiny bubbles of gas begin to rise through the liquid, 
producing a froth on its surface. If, now, the stopper of the flask 




9 8 The Living Plant 

be provided with an outlet tube bent over to end at the bottom 
of a vessel of clear lirnewater, the gas will come bubbling up, and 
will soon turn the limewater milky, thus proving its identity. 
And when the fermentation is ended the liquid left in the flask 
has always that "sourish" smell distinctive of the presence of al- 
cohol, which, indeed, can be separated for testing by distilling 
the liquid. As to its quantity, however, it is important to know 
that even when all the conditions for fermentation are most 
favorable and the sugar is present in plenty, the Yeast neverthe- 
less does not form more than a limited quantity of alcohol, 
(about ten per cent of the liquid in round numbers), for then the 
plant is rendered inactive and may finally be killed by the very 
alcohol which it has produced. 

Such is the process of fermentation, which, as everybody knows, 
is vastly important in the arts. Sometimes it is used for the sake 
of its carbon dioxide and sometimes for the sake of its alcohol. 
The conspicuous case of the former is found in the making of 
bread, where the carbon dioxide released from the growth of the 
yeast cells throughout the mass of the dough, forms the cavities 
by which it is lightened and raised. When everything goes as it 
should, the alcohol evaporates in the baking, but sometimes 
it does not, and then the bread goes "sour." Of course other 
methods of raising bread are in use, notably by aid of gases re- 
leased in the dough from chemical action between the constit- 
uents of suitable "baking powders," or other substances, and 
also by use of air blown into the dough; but yeast fermentation 
is much the most used of the methods. But far more extensive 
is the employment of fermentation for the making of the various 
kinds of alcoholic liquids. When the sweet juice of the grape is 
allowed to ferment (by action of yeast blown as spores through 
the air to the fruits), the carbon dioxide escapes to the air, and 
the remaining admixture of alcohol, water, and flavors we call 
wine. When the sweet pulp of the germinating grains of barley 
is allowed to ferment (by Yeast which is added for the purpose), 



The Kinds of Work That Are Done by Plants 99 

we give the name beer, " lager beer/' to the liquid resulting. 
And innumerable other sweet juices and saps are fermentable, 
with resulting formation of alcoholic beverages, which are so 
many and diverse in kind that most nations have each some 
favorite one of its own, the differences between them being due 
in the main to various flavoring materials originally present with 
the sugar. None of these fermented liquids, however, are ever 
stronger in alcohol than the ten per cent, or thereabouts, whicli 
the Yeast can yield before it is killed. The stronger liquors are 
obtained by an additional and very different kind of operation, de- 
pending on the fact that alcohol boils at a much lower temperature 
than water (78C, or 172F as compared with 100C or 212F). 
For this reason a fermented liquid, if heated above 78 but 
under 100, gives off its alcohol (though also with some water)' 
as vapor, which can be conducted away, cooled and collected 
as a strongly alcoholic liquid. The process is called distillation, 
and in this way are made the stronger alcoholic drinks, brandy, 
whisky, rum, gin, and all the remainder of this precious rogue's 
gallery, their peculiar flavors and colors being due to particular 
substances, sometimes naturally present and sometimes purposely 
added, in the juices from which the alcohol is fermented. It is by 
repeated distillation of the fermented juice of germinating corn 
that the strong alcohol of commerce is made, and this when mixed 
with a little of the poisonous wood alcohol to make it undrinkable 
becomes the " denatured alcohol" of the household and the chaf- 
ing dish. 

We turn now to the chemistry of fermentation, which is simple. 
It is grape sugar which is fermented, for other sugars or starches 
are first changed to that form or its equivalent. Therefore we 
have this expression, 
In fermentation C 6 H 12 O 6 forms C0 2 and C 2 H 6 

grape sugar carbon dioxide alcohol 

This statement can be given an exact chemical form in this 
way, 



ioo The Living Plant 

C 6 H 12 6 = 2 C0 2 + 2 C 2 H 6 O 

And this equation expresses exactly the known facts of the 
process. 

What now is the meaning of fermentation, and why does the 
Yeast do it? Nowhere in Nature, so far as I can find, excepting 
in the case of humanity, is there even the least evidence that any 
kind of organism ever does anything whatever for the sake of 
service to any other kind. We should not expect to find, accord- 
ingly, that the Yeast makes the carbon dioxide and alcohol for 
any disinterested or philanthropic purposes, not for providing 
thrifty housewives with light bread or their shiftless husbands 
with strong drink, and we turn to seek some desirable object of 
its own to which the use by mankind is purely incidental. But 
of course, the reader has inferred the explanation before this, 
fermentation is simply the Yeast's respiration, the source of its 
power for growth and other work that it does. And the explana- 
tion of so peculiar a form of respiration is well known. Living im- 
mersed in a liquid, the Yeast cannot obtain respiratory oxygen 
from the air, and must take it from some other source. Only one 
source is available. Locked up in the molecule of sugar is some 
oxygen brought into it with the hydrogen, which holds it away 
from the carbon, as the formula C G H 12 6 suggests. But the Yeast 
plant, absorbing the sugar into its body, shatters the molecules 
(by means of a peculiar agency called an enzyme soon to be 
described), and allows the carbon and oxygen in the fragments 
to unite with one another; this produces the usual result, a 
copious release of energy which the Yeast at once utilizes for its 
growth, while of course the resulting carbon dioxide is thrown off 
into the liquid. This is the object, or meaning, of fermentation; 
to secure a union of carbon and oxygen for the sake of the energy 
which is always thus released. As to the alcohol, that is simply 
the remains of the shattered molecule; it is a chemical fact that 
the number of atoms of carbon, hydrogen and oxygen which hap- 
pen to be left after the carbon dioxide is formed, fall naturally 



The Kinds of Work That Are Done by Plants 101 

into alcohol, and the Yeast plant cannot help it. That is why the 
Yeast produces the poisonous alcohol, despite the suicidal char- 
acter of the proceeding. The Yeast, however, can respire in no 
other way, and with commendable philosophy, prefers a short 
life, even at the risk of an alcoholic grave, to no life at all. Yet 
in fact the case is not really so bad, for the alcohol is very volatile, 
and in Nature commonly evaporates as rapidly as formed; and 
even when not, the drying up of the liquid and spore-formation 
allow the yeast to escape and renew its activity at another time 
and place. If the Yeast plant had nothing to do but respire, 
the sugar would all be converted to carbon dioxide and alcohol, 
which are probably the sole products of its respiration. But the 
Yeast must also make new substance, protoplasm and walls, 
for which purpose it uses some of the sugar in a different way, 
along with other substances, and thereby develops incidentally 
a small percentage of by-products, glycerin, acids, etc., the pur- 
suit and capture of which affords a fine joy to the special student 
of chemistry, especially if some student of biology has previously 
told him that carbon dioxide and water are the " products of fer- 
mentation." 

Alcoholic fermentation caused by Yeast is the most typical 
and familiar kind, but other sorts occur, caused by germs (Bac- 
teria), or Molds. Thus the souring of milk, the ratification 
of butter, the genesis of vinegar, and even the development of 
distinctive flavors in ripening cheese, are products of fermenta- 
tions, caused in their respiration by various organisms. As these 
cases illustrate, the secondary products need by no means con- 
sist only of alcohol, but can include substances of the most diverse 
chemical natures. All that is requisite is that carbon and oxj r gen 
shall be allowed to unite; the matter of the particular compounds 
is secondary. 

If any doubt could exist that fermentation is simply the respir- 
ation of the Yeast plant, it would vanish before the remarkable 
fact that an exactly intermediate step is known between the 



102 The Living Plant 

respiration of the higher plants and typical fermentation. Ideally, 
in the respiration of the higher plants, the oxygen absorbed and 
carbon dioxide released are equal in volume, but often they are 
not. Thus, some kinds of seeds, like Peas, if shut away from 
oxygen, can release plenty of carbon dioxide without absorbing 
any oxygen at all; and analysis of the seeds then shows the pres- 
ence of alcohol. In other words, these Peas, like the Yeast plant, 
can cause fermentation (though in limited degree) of some of 
their own substance; and there is no doubt that it represents the 
form of respiration to which the seeds resort when no oxygen 
from the air is available. This form of fermentation is called 
in the Peas, and the other plants which make use of it, anaerobic, 
or intramolecular , respiration. 

There remain two other forms of fermentation so important 
as to require a separate treatment. One is decay, or putrefaction, 
which is really the fermentation of dead plant and animal sub- 
stances by Bacteria, or germs. Bacteria are plants even smaller 
and simpler than Yeasts. The products of their respiration and 
growth are most diverse, including not only carbon dioxide and 
water but various other gases, some of which possess those very 
vile odors distinctive of rotting organic matter. When the de- 
caying substances are complex, e. g., flesh or other proteins, certain 
Bacteria ferment them to simpler sorts, other kinds to simpler 
still, and so on, until they are finally reduced, as in ordinary respir- 
ation, to carbon dioxide and water, and such other elemental 
substances, (e. g., nitrogen) as may also have entered into their 
composition. All decay is simply a form of fermentation, that is 
respiration, by Bacteria, or, in some cases, by simple Molds. 

Another phase of the same phenomenon is involved in those 
deadly diseases which are caused by Bacteria, Asiatic Cholera, 
Tuberculosis, Diphtheria, Typhoid, Lockjaw, and a number of 
others. It is a popular belief that Bacteria produce their effect 
in disease by destroying the tissues, or, as a plain-spoken student 
of mine once expressed it, they "chew you all up inside." That 



The Kinds of Work That Are Done by Plants 103 

belief is far from the truth, for what happens is this. The Bac- 
teria, in order to obtain energy and material for their own pro- 
cesses, act on the tissues or the blood in just the same way that 
Yeast acts on the sugar, likewise forming incidentally in the act 
various accessory substances. Now some of these substances, 
bearing much the same relation to the Bacteria that alcohol 
does to the Yeast, are those alkaloids or ptomaines which happen 
to be violently poisonous to man, and it is these poisons, and not 
the Bacteria directly, which are the cause of his death. At least 
they are the cause of his death if they are formed more rapidly 
than his system can antagonize them, for the body has a wonder- 
ful power of forming antagonistic chemical substances, or anti- 
bodies, which neutralize these poisons, which antibodies, by the 
way, can be made to form in the body, or even can be injected 
as antitoxins, ensuring immunity against some diseases. These 
deadly diseases are therefore an incidental result of the respiration 
and growth of Bacteria which are leading their own lives in their 
own way, as oblivious to any harm they may do as is the Yeast 
to the benefit it confers. 

It is not only true that fermentation, decay, and some disease, 
are caused by the activity of Yeasts, Molds, and Bacteria, but 
the converse is equally well-known, that those processes occur 
through no other agency and can be prevented entirely by killing 
these organisms. This can be done by heat, poisons, certain 
strong solutions, or even, in some cases, bright light; and such is 
the basis of the various sterilizing and antiseptic processes so 
familiar in the household, the arts, and in medicine. 

We can now express these later facts in another of our verities 
as follows; all fermentation and decay, and some phases of dis- 
ease, are forms of the respiration of simple organisms which thereby 
destroy organic matter by reduction back to the carbon dioxide, water, 
and other elements, from which it was originally built up. 

It is thus evident that all of the carbon dioxide and water 
built into plant substance by photosynthesis, are ultimately re- 



104 The Living Plant 

leased again by respiration or decay. A quantity, rather small, 
of the earth's supply of carbon dioxide and water is therefore 
always locked up in plant and animal substance; but though the 
quantity is approximately constant the precise molecules are 
constantly changing, and with the changes go those transforma- 
tions of energy which are the principal manifestation of life. And 
if the question be asked, why are not more of the carbon dioxide 
and water of nature locked up in plant and animal substance, 
that is, why are there not more and larger plants and animals on 
earth, I think the answer is easy. There do already exist upon the 
earth all of the plants and animals, and as big ones, as the physical 
conditions permit. As to plants, every spot on the earth that 
can maintain plant life at all is bearing all the plants it can sup- 
port, and these plants are just as big as the physical conditions 
permit them to grow. As to animals, they are dependent upon 
plants for their food, and it is evident that there is available for 
their use only the surplus of food produced by plants over that 
which these need for themselves, and animals are just as abun- 
dant and big as that surplus can support. 

Thus, these apparently very complicated processes of photo- 
synthesis and respiration, like many another and probably like all 
of the physiological processes in plants and in animals, can be 
reduced to a basis of pure physics and chemistry. And we shall 
learn later, in our chapters on Irritability and on Growth, that 
we have a good explanation of the orderly sequence and regular 
connection of the processes in their linking up together through 
their interactions as stimuli. Is there then, nothing in the plant 
except the interactions of chemistry and physics? Let the remain- 
ing pages of this book give their testimony before we attempt the 
answer. 



CHAPTER V 

THE VARIOUS SUBSTANCES MADE BY PLANTS, AND THE 
USES THEREOF TO THEM AND TO US 

Metabolism 




N chapter two of this book it was shown that plants 
manufacture grape sugar in their lighted green leaves; 
and I said it would later be proven that this sugar rep- 
resents a basal food substance out of which, with sundry 
minor additions, plants build all of their other materials. The 
time has now come for this demonstration, to which, as a sub- 
ject possessing perhaps more importance than interest, I bespeak 
the reader's somewhat spartan attention. Since all of the sub- 
stances constructed by plants have a meaning in their vital 
economy, I might also have entitled this chapter "on the various 
uses that plants make of their food," in which case I should 
have to commence with a review of respiration, for that is the 
most important of the uses of food. The others here follow in 
an order determined chiefly by the chemical nature of the sub- 
stances concerned. 

The number of substances constructed by plants is verily 
legion, for the vast variety of foods and fabrics, drugs and dyes, 
and other materials yielded by them to us is only a small pro- 
portion of those which they actually make. Fortunately, how- 
ever, for our limited comprehensions, those which are really 
important are few, and moreover, they fall into somewhat defi- 
nite classes. Since the subject is new to most persons, I will give 
these classes in synopsis as a kind of table of contents to this 
chapter. They are these: 

105 



io6 The Living Plant 

Class I. The BASAL FOOD, or PHOTOS YNTHETIC SUGAR; the substance first 
formed in lighted green leaves; composition CoHi20o. 

Class II. The FOODS, active and reserve, and the SKELETON; chemically 
called CARBOHYDRATES, with a composition identical with or 
readily .transformable from that of the photosynthate, viz., 
CeHiaOc, or Cial^On, or (CuHioO 5 )/i. 

Class III. The SECRETIONS; various non-nitrogenous substances, mostly of 
special ecological functions, DERIVATIVES OF CARBOHYDRATES 
and containing the same elements, but in markedly different 
proportions, and hence collectively expressible only in the form 

C n H n O n . 

Class IV. The NITROGEN-ASSIMILATES, chemically called AMIDES; inconspic- 
uous but important substances containing the elements of the 
photosynthate with the addition of nitrogen, and forming the 
transition from Class I to Class VI; collectively expressible 
only as C n H n O n N n . 

Class V. The PRINCIPAL POISONS, chemically called ALKALOIDS; containing 
(as a rule) the elements of the Amides but in different pro- 
portions, substances of uncertain meaning, and collectively 
expressible as C n H n (O n ) N n . 

Class VI. The FLESH-FORMERS, chemically called PROTEINS, contributing 
to the formation of protoplasm and consisting of the elements 
of the Amides with the addition of sulphur and phosphorus, 
and collectively expressible only as C n H n O n N n S n (P n ). 

Class VII. The REGULATORS OF METABOLISM, called ENZYMES, substances 
of unknown composition, but supposed to be proteins, possess- 
ing remarkable properties of causing chemical transformations 
in other substances. 

Class VIII. LIVING PROTOPLASM. 



Class I. The Basal Food, or Photosynthetic Sugar 

This substance needs no introduction to the reader of the earlier 
parts of this book; but for others it may be characterized as a 
sugar made abundantly in the lighted green leaves of plants from 
carbon dioxide and water, and forming the foundation of all 
organic substances. It belongs in a class by itself only because 
of its unique mode of formation and function, for chemically 
it belongs in the second class, being nothing other than a mixture 
of the grape and fruit sugars next to be described. 



The Various Substances Made by Plants 107 

Class II. The Food and Skeletal Substances, or Carbohydrates 
Grape Sugar. This substance is formed abundantly in green 
leaves as the photosynthate, and is common in nearly all parts 
of all plants. It is, however, much less known than its import- 
ance would imply, because it has no prominent economic uses, 
and exists in the plant only in solution in the sap of the cells, 
which therefore display through its presence no more striking 
appearance than that represented in the accompanying example 
(figure 33). However, it sometimes ac- 
cumulates considerably in fruits, which 
it helps to make nutritious and attract- 
ive to animals in connection with dis- 
semination, a subject to be later dis- 
cussed in a special chapter devoted to 
that subject; and in grapes, especially, 
it is so plenty that it crystallizes out 
when they are dried, forming the soft 
sugar abundant on some kinds of raisins. y IG . 33. Appearance m opti- 

Tt^ rrmnv qnH on^v trqnciffvrrnqtirm^ intn cal section ' highly magnified, 

its many ana easy transformations into of a ( . dl in which sugar is 
other substances will be traced in the stored in thc sa i>- . . 

Inside thc wall is a lining of hv- 

followillg pages. It haS, however, a ing protoplasm which encloses 
T . . T . .* ,1 the large sap cavity wherein 

second origin and significance in the i s wate r containing t ho dis- 
plant, for it is that into which many other solvcd sugar - 
substances are converted in digestion, as we shall presently learn, 
and is the commonest form in which substances are translocated 
through the plant. It is white in mass, looks amorphous and not 
crystalline to the eye, is sweet to the taste, though much less 
sweet than cane sugar, and is the easiest of all sugars for Yeast 
to ferment. It is interesting to know that it has been made 
artificially in the chemical laboratory. Chemically its correct 
name is dextrose, though often also called glucose, and its formula 
is C 6 H 12 6 . 

Fruit Sugar. This substance is extremely like grape sugar, with 
which until lately it was more or less confounded, and with which 




io8 The Living Plant 

it occurs in the various roles above mentioned for grape sugar. 
It is sweeter than grape sugar but ferments less easily. Chem- 
ically it is called fructose, and has the formula C 6 H 12 6 , differing 
from grape sugar not in the kind or number of atoms entering into 
its composition, but in the arrangement of these within the mole- 
cule, as best demonstrated by physical tests with polarized light. 

Cane Sugar. This substance is perfectly familiar to everybody, 
for it is the granulated sugar of the table. It is widely spread 
through plants dissolved in the sap, and accumulates in some kinds 
so abundantly as to form a reserve supply of food for them, and 
a store upon which animals, inclusive of man, are accustomed to 
draw for their needs. This accumulation occurs conspicuously in 
the Sugar Cane and the Sugar Beet (both of which plants have 
had their percentage of sugar immensely increased by cultivation), 
in the Maple tree, and in a few other less conspicuous plants, 
while it is common as well in ripening fruits. Chemically cane 
sugar is called sucrose, and has the formula C 12 H 22 11 . It is 
built up by living protoplasm from photosynthetic sugar through 
this simple step, 2 C 6 H 12 6 -H 2 (water) ^C^H^On; and it falls 
back by a reverse process to a molecule of grape sugar and one of 
fruit sugar. This latter step actually occurs in the ripening of 
fruits, in cooking, and in digestion; and it is, therefore, as grape 
sugar or fruit sugar that cane sugar is finally incorporated into 
both the plant and the animal body. 

In addition to these sugars, there are others of rarer sort de- 
scribed in the technical books, all closely related and more or 
less intertransformable into those we have mentioned. Such are, 
for example, maltose, mannose, galactose, arabinose, xylose, 
fucose. I am very well aware that these names will have no great 
attraction for the reader, but I take somewhat the same satis- 
faction in their recital that Homer derived from the roll of his 
heroes, whom also he mentions but once. 

Starch. This substance is perfectly familiar to everyone as 
common laundry starch, and especially as flour, which is mostly 



The Various Substances Made by Plants 109 

composed of it. Occurring as a rule in tiny white grains scattered 
widely through all kinds of tissues, it collects in some organs, 
which swell very greatly for its reception. Such is the Potato, 
which is simply a starch-storing underground stem: the Sweet 
Potato, a starch-storing root : bulbs, which are masses of starch- 
filled leaves: and most seeds, including all of the grains, which 
contain copious starch either inside or around the embryo. In 
all of these cases, starch presents a characteristic homogeneous 
firm whitish appearance, contrasting markedly with the soft 
translucent aspect of structures in which the food is stored up as 
sugar, e. g., the Sugar Beet, Sugar Corn. It happens, however, 
that its presence can be detected in a very conclusive way, namely 
by the deep blue color it assumes when touched by a solution of 
iodine, as the reader already has learned, and as he can easily 
prove for himself by applying a little of the tincture of iodine to a 
lump of starch from the laundry box, or to a disused cuff, or to 
water in which some starch has been scraped, and heated until 
it forms a fine paste. The test is one of the most satisfactory 
and important in all organic chemistry, and so delicate that, by 
its use with the aid of the microscope, one can detect even the 
minutest quantities of starch in the tissues of a plant, where it is 
sometimes distributed with a curious and beautiful geometrical 
exactness. It is necessary to warn the experimenter, that in 
living tissues, however, the test often works rather badly, because 
iodine penetrates active protoplasm very slowly. 

Starch, when it accumulates in the plant, serves as a store of 
reserve food upon which the plant can draw when it starts new 
growth; and starch is by far the most common and abundant of 
plant foods. Moreover, it serves equally well as a food for an- 
imals, which, accordingly, rob the plants; and these are there- 
fore obliged as a whole to make a huge surplus in order to keep 
any at all for themselves. The importance of starch as food for 
man is evident when one recalls that Wheat, Corn, Rice, Barley, 
Rye, grains, which constitute the principal food of the great 



no The Living Plant 

majority of the human race, are composed almost wholly of 
starch. 

Chemically, starch has the formula (C 6 H 10 5 )n. It is formed 
apparently thus, from dextrose, C C H 12 6 , water, H 2 0, is with- 
drawn, leaving C 6 H 10 5 ; this substance does not occur in this 
form in the plant, but the molecules immediately aggregate them- 
selves (chemically, polymerize), to a considerable but unknown 
number, expressed by the letter n, into compound molecules. 
Starch is made up in this way from dextrose, and it is of interest 
to note that a corresponding substance made from fructose occurs 
as a reserve food dissolved in the sap of the swollen roots of some 
Composite plants, where it is called inulin. The formation of 
starch has never been effected artificially outside of plants, and 
in them it takes place only inside of those living protoplasmic 
bodies called plastids, which include chlorophyll grains and which 
are to be described more fully in the next chapter. The re- 
conversion of starch to dextrose is effected through the action of 
diastase, one of those remarkable chemical agents called en- 
zymes, which we are presently to study; and this is exactly what 
happens in the digestion of starch in both the plant and animal 
body. Indeed, this digestion can be carried on experimentally 
and very easily in a test-tube by action of diastase bought from 
any chemical supply company, the disappearance of the starch 
being proven by use of the iodine. 

A fact of another kind about starch should be noticed at this 
place. Even to the unaided eye it looks granular in texture, while 
the microscope shows that it really is composed of definite grains, 
which, moreover, display a remarkable structure. If a section 
be cut from the interior of a potato, for instance, and magnified, 
the cells are found to present an aspect well shown in the typical 
example here pictured, (figure 34). Within each cell are numerous 
solid grains, various in details of their shapes, but all possessing 
in common a focal spot near the smaller end, around which are 
excentrically-arranged layers (figures 34 and 35). Starches from 




fied, from a Potato, show- 
ing the concentrically-lined 
starch grains embedded in 
living protoplasm. 



The Various Substances Made by Plants in 

other plants are of different aspect, as our plate so clearly illus- 
trates (figure 35) ; but each kind exhibits characteristics peculiar 
to itself, and in general it is true that no two species of plants 
have grains exactly alike, while each 
species has a kind distinctive of itself. 
This fact has a practical value, because 
experts with the microscope can thus 
learn to recognize the starches of dif- 
ferent plants at sight, and by this means 
can detect adulterations in starchy foods 
or drugs. Biologically, also, this indi- 
viduality of the starches is of very great 

interest, for it gives us a clear case in FIG. 3-1. A ceil, highly magni- 
which a well-developed specific character 
exists without any regard to utility; for 
even the most radical adaptationist would 
hardly consider the forms of the deeply-buried and invisible 
starch grains as useful in adapting the species to its environment. 
And if an internal specific character can be useless, what need to 
try to explain every external specific character as necessarily 
useful? I am very well aware that this little digression will seem 
without point to most of my readers, but I pray them to have 
patience a little, for I have a good object. I am calling their 
attention when I can to certain data which will later be useful 
when we come to consider the subject of evolution. 

Cellulose. This substance is vastly abundant and prominent 
in plants, for it is the material out of which they construct the 
walls of their cells and therefore their entire firm skeletons. 
The reader can obtain a good idea of pure cellulose by recalling 
the fibers of cotton, the pith of woody stems, or some of the pur- 
est unstarched paper, such as the filter-paper of the laboratories, 
all of which exhibit the distinctive cellulosian qualities of tough- 
ness, elasticity and transparency. In some plants also, it is 
stored up as a reserve food in the seed, when it appears as an im- 



112 



The Living Plant 



mense thickening of the cell- wall (figure 36). A conspicuous 
case is the Ivory Palm, which has seeds so hard as to constitute 
a substitute for ivory in the making of buttons and other bijou- 
terie, while the seed of the Date owes likewise its stony hardness 
to the same material. Though so hard, this cellulose is easily 
digested to sugars by the action of suitable enzymes, and the pro- 









FIQ. 35. Typical grains of a dozen different kinds of starches, highly magnified. The 
kinds, in order of arrangement in this picture are; 

Potato Maranta Pea Hyacinth 

Wheat Oats Sago Smilax 

Canna Corn Bean Oxalis 

cess is applied commercially to ordinary wood in the manufacture 
of wood alcohol. Naturally, the very cells which make cellulose 
have the power to digest it away once more where needful; and 
this is why cell-walls, even when well grown, can become perfo- 
rated, absorbed, split, or even re-adjusted in such a way that they 
seem to have slid upon one another. 



The Various Substances Made by Plants 113 



Chemically, cellulose is related to grape sugar and formed there- 
from in much the same way that starch is, its formula being the 
same as that of starch, (C 6 H 10 O 5 )n, with the n, however, represent- 
ing a different but unknown value. 

Although cell-walls when young consist only of cellulose, in 
some structures they become penetrated later by other materials 
which are probably formed by alteration of 
the cellulose itself, and which give new proper- 
ties to the walls. Thus, it is a stiffening sub- 
stance called lignin, added to cellulose walls, 
which converts them into wood, and also 
forms other hard tissues, such as the shells of 
nuts; while a very different substance, cutin 
or suberin, makes the walls thoroughly water- 
proof, as they are in all cork, and in the thin 
waterproof epidermis which ensheaths the en- 
tire plant. 




FIG. 36. A cell with 

An alteration of the cellulose of parts of four others, 

, , . , , ., ., from the interior of 

another kind produces the mucilaginous ma- the nut of ivory Palm, 

showing the walls im- 
mensely thickened by 
deposition of layers 
of cellulose, through 
which run canals per- 
mitting acontinuity of 
protoplasm from one 
cell cavity to another. 



terial displayed when some seeds (e. g., those 
of the Flax), are placed in water, or when 
fallen leaves turn gummy on sidewalks in wet, 
warm, autumn weather; and such also is the 
origin of the mucilage or slime found in des- 
ert plants on the one hand and water plants on the other, with 
peculiar functions in those plants to be later considered. 

There are highly consequential facts of another kind about 
cellulose whether lignified or not. It burns readily in presence 
of oxygen, being converted back in the process to carbon dioxide 
and water, the very substances from which it was originally made. 
When, however, it is subjected for a long period of time to pres- 
sure and heat, gradually it undergoes definite chemical changes 
through which its hydrogen and oxygen are removed, leaving 
behind the solid and non-volatile carbon. This is exactly what 
has happened in the case of the plants which grew of old time in 



ii4 The Living Plant 

the swamps of the Coal Period; their walls, losing the oxygen and 
hydrogen, have become proportionally richer in carbon and in- 
cidentally darker in color, passing gradually through stages repre- 
sented by peat, lignite, soft coal and anthracite, which latter is 
almost entirely carbon. It is thus that our beds of coal have 
been formed. Somewhat the same thing occurs, through the action 
of heat, in the charring of wood, and a similar process produces 
the black humus of good soils from roots and the like. When the 
carbon of coal remains yet longer exposed to suitable conditions, 
it becomes graphite or black lead, while if crystallized it forms 
diamond, the end of the series. And it is interesting to note in 
this connection that we do not yet know any natural way by which 
pure carbon can be isolated from oxygen without photosynthesis 
constituting a step in the process. If one were to burn the dia- 
mond, he would form carbon dioxide again, and thus close the 
chain of transformations through which the carbon has gone since 
it was absorbed from the air by a living green plant long ages ago. 
Arid as to this burning, it is interesting to reflect that the heat and 
light released in the combustion of coal is energy that was rendered 
latent by the photosynthetic dissociation of carbon dioxide when 
the coal was first formed as a photosynthate; it has been kept 
stored all this time in the unsatisfied affinity of its carbon for 
oxygen; and when released in our midwinter fires, it is really the 
heat and the light of the ancient carboniferous sun that is warm- 
ing and cheering us. 

Gums. These are solid but very elastic sweet substances, of 
which gum arabic, used in gurndrops and on postage stamps, 
is the most familiar example; the gum of cherry trees is another, 
and the substance of marsh mallows another, though the spruce 
gum, of the woods and the schoolroom, is quite different as will 
be noted below under resins. These gums are accumulated in 
rifts of the tissues of some trees, but it is not at all clear why the 
plant should make them, though apparently they serve at times 
as reserve food. Chemically they have the formula (C 6 H 10 5 )n, 



The Various Substances Made by Plants 115 

the same as that for cellulose and starch, but with n meaning 
another figure; and they are formed no doubt from grape sugar, 
(probably via the mucilaginous modification of cellulose men- 
tioned in the preceding paragraph), to which they are readily 
digested back by both plants and animals. 

Fruit-Jellies. These substances are familiar to all housekeepers 
as the jelly which forms when fruits or vegetables are cooked 
(e. g., grape jelly, orange marmalade, pumpkin preserves), though 
it must be remembered that gelatine, from which the jellies of the 
tea-table are made, is an animal product. In the living plant 
they are solid, being insoluble in cold water; but they are dissolved 
by hot water, which explains why they appear after cooking. 
They represent, it is believed, another form of reserve food. Chem- 
ically they are known as pectins, and they have also the same 
general formula as starch (C 6 H 10 O 5 )n. They are formed without 
doubt from grape sugar to which they are easily digested back. 

In reading this account of these various carbohydrates, two 
questions will inevitably arise in the mind of the reader. First, 
he will ask how it is possible that substances with properties so 
different as those of starch, cellulose, gums and jellies can have 
the same chemical composition. The answer is this, that on the 
one hand the letter n in these formulae represents without doubt 
a different number in each case, and hence the composition is not 
really identical, while on the other, even an identical formula 
can be associated with very different properties, because the 
properties depend not only on the elements present, but upon 
the way these elements are arranged in the molecule; and they 
can be arranged in very different ways. The differences between 
grape sugar and fruit sugar are wholly of this latter kind. The 
second question the reader will wish answered is this, why do 
some plants store up their reserve food in the form of sugar, some 
as starch, others as cellulose, and others as oil, soon to be men- 
tioned. This question we cannot yet answer with certainty, but 
probably the general explanation offered in Chapter III for the 



n6 The Living Plant 

diverse ways in which plants develop the same organs, applies to 
the present matter, also, namely, the plant makes the form of 
food easiest chemically for it to construct, provided of course 
there is no ecological reason for making one kind rather than 
another. 

Class III. The Secretions, or Derivatives of Carbohydrates 

So heterogeneous are these substances in composition, proper- 
ties, and uses, that they are held in one class by hardly any 
stronger bond than that, while including the elements of Class II, 
they do not belong therein. Nor is the name which I give them a 
good one, for they include some things which are not truly secre- 
tions, while not all of the secretions are included in this class; but 
I can think of no better general designation. The principal 
members are the following. 

Plant Oils. These are of two distinct kinds. First, are the fixed 
oils, which are properly plant fats, familiar to us in the various 
oils used in food or in medicine, notably olive oil, castor oil, cot- 
ton seed oil. They occur rather widely scattered in plants, as 
tiny isolated drops, scattered through the protoplasm; but they 
accumulate in quantity in many kinds of seeds, including nuts, 
to which they give a distinctive oily luster, and in which they act 
very obviously as a reserve food for the use of the embryo in 
germination. A reason why oil is stored in seeds more frequently 
than elsewhere has been found in a linking of two facts; first, 
food value for food value, oil is a much lighter substance than any 
other kind of food stored by plants; and second, the seeds storing 
it are mostly disseminated by the wind and hence need to be kept 
just as light in weight as possible. And with these oils as with 
other substances, good food for plants is good food for animals 
also, the food needs of both being closely alike. Chemically 
these fats are rather complex, a typical formula being C 57 H 110 6 , 
which shows that they are markedly poor in oxygen; and herein 
lies the reason why plenty of fresh air is needed for their assimila- 



The Various Substances Made by Plants 117 

tion by man. They are formed in living cells from starch, and 
therefore ultimately from grape sugar, to which they can be 
changed back in germination and digestion by the action of suit- 
able enzymes. 

Related to the fats in some respects, though to the later- 
described proteins in others, are the lecithins, widely distributed 
in plants, and possessing a considerable interest as the probable 
basis for the formation of the vastly-important and complicated 
substance chlorophyll, the composition of which, aside from the 
presence of carbon, hydrogen, oxygen and phosphorus, is still 
rather uncertain. 

The other kind of oils, the ethereal, essential, or volatile 
oils, are very different in composition and meaning. They are 
familiar to us chiefly in the fragrant oil of lemon and oil of cloves, 
and are the causes also of the odors, sometimes fragrant and 
sometimes acrid, of many kinds of leaves (e. g., Lemon Geranium) 
when cut or crushed; and they cause likewise the fragrance of 
flowers and fruits. Camphor, and some other aromatic materials 
are related substances. They are not food products, as the fats 
are, but serve mostly ecological uses, either in connection with 
the protection of plants against insects or Fungi, or for the at- 
traction of animals in connection with dissemination of seeds 
and cross pollination of flowers, as we shall later consider in detail 
along with those respective subjects. They are stored as a rule 
in special receptacles or glands, often of considerable size (figure 
37). Chemically they are most diverse, some of them consisting 
only of carbon and hydrogen, approaching near to the formula 
C 10 H 16 . Little is known as to their exact mode of formation. 

It is a non- volatile oil (called toxicodendrol), which is the 
poisonous susbtance in the Poison Ivy; and the fact that it is a 
non-evaporating oil explains why it is so very difficult to remove 
from the skin, and why it persists in plants which are long dead 
and dried. 

Plant Adds. These are agreeably familiar to us as the sub- 




n8 The Living Plant 

stances which give the pleasant acid taste to fruits. Thus, malic 
acid gives the tart taste to apples and currants, citric acid to 
lemons and oranges, tartaric acid (from which cream of tartar 

is made) to grapes. In all of these 
cases there is a reason, as our 
chapter on Dissemination will 
show, why these fruits should be 
eaten by animals, to which the 
acids certainly serve to render the 
fruits more attractive. On the 
other hand tannic acid, which oc- 
curs in the bark of many plants, 
(and from which man extracts it 

FIG. 37.-A gland, highly magnified, f r tanning leather), has an as- 
formed by a fusion of several cells tringent taste unpleasant to ani- 
containing a large drop of an ethereal " A 

oil, as seen in a cross-section of a leaf mals, against which, accordingly, 

of Dictamnus Fraxinella. . , 

its presence has some tendency to 

protect the plant tissues. These acids, which all occur in solu- 
tion in the sap, have a comparatively simple composition, the 
formula of malic acid, for example, being C 4 H 6 5 . Their mode 
of formation is not entirely understood. 

Plant Waxes. These occur chiefly on the surface of plants, 
where they constitute the bloom, commonly of bluish color, which 
is familiar upon plums and some leaves. On the dry berries of 
the Bayberry, a common plant of the coast, a wax accumulates 
in such quantity that in early days it was gathered and used for 
the making of candles. In general the waxes seem to render 
plants immune against wetting, after the manner of the oil on 
the back of the proverbial duck, the disadvantage of the wet- 
ting being this, that the water would clog the stomata, and hence 
prevent the passage of gases that are needed in photosynthesis. 
If, now, the reader should ask me why, when the wax is thus of 
advantage, so many plants do not have it, I would answer by 
asking in turn why it is, that, if riches are such an advantage (or 



The Various Substances Made by Plants 119 

at least are commonly thus reckoned), so few men possess them. 
The reason I take to be fundamentally the same in both cases; 
some kinds never get the right start towards constructing them, 
or else have not the capacity to manufacture them. Chemically 
the waxes are very closely related to the oils, and no doubt are 
built up in the same general way. 

Resins. Under this name falls a variety of substances of which 
typical examples are familiar in the balsam of the Fir and the 
Pine; in spruce gum; and in rosin; myrrh and frankincense arc 
others; and much of the milky juice (or latex) of plants, from which 
the rubber of commerce is made, is composed of resins or closely 
related substances. Chemically the resins are most diverse, and 
their mode of origin is as little understood as is their function in 
the plant. They are usually accumulated in special passages, 
from which they sometimes flow out at a break (e. g., in Pines), 
in a way to suggest that they serve as a temporary salve, a kind 
of first aid to an injury. At times they appear to be utilized as 
food, which is likely enough, since there is every reason to sup- 
pose that plants, precisely like animals, when driven by hunger, 
will resort to the use of materials which they would otherwise re- 
ject with disdain. 

Glucosides. These substances are more interesting than con- 
spicuous, the most familiar being that called arnygdalin, which 
gives the bitter taste to seeds of almonds and apples; while the 
peppery taste, so common in plants of the mustard family, is 
also due to a glucoside. Their meaning in the plants is not known, 
although they may find some incidental service in protecting 
against animals the parts which possess them. With the gluco- 
sides belong also some of the brightest coloring matters produced 
by plants, including the red dye madder and the blue dye indigo. 
Here also comes erythrophyll (called also anthocyan), that red 
color with which we have made pleasant acquaintance already as 
giving brilliant hues to ripened fruits, and the glory to the fo- 
liage of autumn. Chemically the glucosides owe their name to 



120 The Living Plant 

the fact that they are compounds of glucose with some one or 
more definite substances, into which they can again be broken 
up. Some of them contain nitrogen, as for instance the amyg- 
dalin above mentioned (its formula is C2QH27OHN), which allies 
them in some measure with the nitrogen-containing substances 
next to be considered, especially the alkaloids. 

Class IV. The Nitrogen-Assimilates, or Amides 

These substances, dissolved in the sap of plants and having 
no particular uses to us, are not commonly known; but they are 
vastly important nevertheless, inasmuch as they constitute the 
connecting step between the carbohydrates and the indispensable 
proteins, soon to be considered. The commonest is asparagin, 
dissolved in the sap of young asparagus plants, from which it can 
easily be crystallized out. Its formula, typical of the group, 
is C 4 H 8 3 N 2 , which shows the presence of the nitrogen along with 
the elements of carbohydrates; and there is no doubt that the 
ultimate source of the materials is the photosynthetic grape sugar 
together with nitrogen from compounds absorbed with water by 
the roots. The amides are not known to perform any special func- 
tion of their own in the plant, and probably find their significance 
simply as a necessary chemical step in the formation of proteins. 

The incorporation of nitrogen with the elements of the car- 
bohydrates is a step of the first biological magnitude, since the 
nitrogen is the most essential and distinctive additional con- 
stituent of the most important of all biological substances, 
living protoplasm. We have already considered, (in Chapter 
II), the source of the plant's supply of carbon, oxygen, and hydro- 
gen, and must now turn aside from our main theme to examine 
the source of the nitrogen supply, a subject all the more important 
because of the fundamental economic bearings it has. Nitrogen, 
it should be needless to recall to the reader, is the colorless gas 
which makes up very nearly four-fifths of the atmosphere; and 
from such an abundance plants ought apparently to have no 



The Various Substances Made by Plants 121 

difficulty in drawing all that they need. As a matter of fact, 
however, the typical plants take no nitrogen at all from the air, 
even starving to death for want of a little while bathed in this 
lavish abundance; and the reason they do not is that they cannot. 
The most prominent characteristic of nitrogen is its chemical 
inertness, or reluctance to enter into combination with any other 
substances, a circumstance, indeed, to which its abundance 
in the atmosphere is due; and its union with oxygen or other 
substances can be effected only by the agency of electric sparking 
machines, or other methods involving the expenditure of high ten- 
sion energy. Now our typical large plants have not in their struc- 
ture any equivalent for sparking machines or other arrangements 
releasing suitable energy, although, as will presently appear, the 
lowly Bacteria seem better provided in this particular. Since they 
cannot make use of the free nitrogen of the air, plants have had to 
resort to the only other possible source of supply, viz., substances 
in the soil containing it already combined, which substances, 
moreover, must be soluble in water to admit of their absorption 
by the roots. The compounds called nitrates best meet these 
conditions, and they, accordingly, are the source of most of the 
nitrogen which, with appropriate intermediate chemical steps, 
is combined with the elements of the carbohydrates to form 
amides. 

If nitrates were as plenty in soils as plants could make use of, 
then our digression in pursuit of this substance could end right 
here. But in fact the nitrates in most soils are so scant that the 
majority of plants live all the time in touch with nitrogen scarcity, 
and this is one of the chief of the factors which limit the luxuriance 
of their growth and expansion. It is, perhaps, worth noting in 
passing, that especial scarcity of nitrogen in some situations 
is correlated with an insectivorous habit in plants which reside 
there, the advantage of this habit consisting in the abundance 
of combined nitrogen obtainable by digestion from the bodies of 
insects. A chief reason for the scarcity of nitrates in the soil lies 



122 The Living Plant 

in that very solubility which renders them absorbable by plants, 
for it leads to their constant drainage away with the superfluous 
water; and were it not for a constant renewal of the nitrate sup- 
ply plant life would soon be starved to extinction. This renewal, 
known as the nitrification of soils, is a matter of such biological 
and economic consequence that we must now consider it with 
some care. 

The natural nitrification of soils takes place in four ways. 
First, there is a constant return of combined nitrogen to the soil 
from the excretions of animals, and the decay of plant and animal 
bodies. Second, a small amount of combined nitrogen is added 
to the soil with the rain which falls during thunder showers, for the 
lightning acts as a kind of gigantic natural sparking machine which 
forces the nitrogen and oxygen of the air into combination; 
thus is formed the soluble nitrous acid, which is caught and taken 
into the soil by the rain. Third, nitrates are constantly though 
slowly added to the soil by the natural decay of the rocks which 
contain them. In moist climates they must drain away about as 
fast as they are formed, but in dry climates the drainage is slower 
than their formation and they accumulate in the soil. This is 
a reason for the richness of the finer soils of the deserts, which 
blossom as the rose when water is added by aid of irrigation. 
Fourth (and far the most important) of the natural methods of 
soil nitrification is bacterial activity. Everybody knows that a 
soil in order to be rich must contain a proportion of humus, 
the material which is dark in color and supplies the open char- 
acter. This humus consists chiefly of decaying vegetable matter, 
which provides both the home and the nourishment for countless 
numbers of tiny organisms, chiefly Molds and Bacteria. These 
Bacteria, popularly known as Germs, are of several kinds, of 
which some, in the course of their own processes, incidentally 
work over the less valuable nitrogen compounds of the soil to 
more valuable ones, while still others, and these the most impor- 
tant, actually force the nitrogen and oxygen of the air to unite 



The Various Substances Made by Plants 123 

into the simple compounds which later are worked up to nitrates 
by the others. It is not yet known how these Bacteria accomplish 
this crucial first step of nitrification, but the source of the energy 
is plain; it is supplied by their intense respiratory power, in which 
they surpass some hundred-fold the larger plants. This fact of 
the nitrification of soils through the activity of Bacteria is one 
of the most important in nature. 

It may here occur to the practically-minded reader to ask 
whether this power of Bacteria to add nitrogen compounds to 
soils cannot be utilized artificially for the enrichment of poor soils. 
It can be, and to some extent, has been; and living Bacteria 
of the suitable sorts have actually been multiplied and distributed 
for trial by our own Department of Agriculture, and have been 
offered for sale to farmers both in Europe and America, though 
the process is not as yet a commercial success. However, in the 
utilization of the nitrifying Bacteria man was long anticipated 
by at least one great group of Plants, the Pea Family, or Legu- 
minosse, the members of which have actually colonized the nitrify- 
ing Bacteria upon their own roots, thus making sure that the en- 
tire product of the Bacteria shall be available to themselves 
without any loss through drainage or use by other plants. Most 
people have seen upon the roots of Peas, Beans, and others of 
this family, the wart-like or pea-like swellings, whose appearance 
is well shown in the accompanying photograph, (figure 38). 
These nodules are residences inside the plants occupied by the 
Bacteria. The connection is mutually beneficial, for the Bacteria 
receive carbohydrates from the green plants which receive nitrog- 
enous compounds from them. It is because of the efficiency of 
this arrangement that the seeds of plants in the Pea Family are 
richer in nitrogenous food substances than any others; and this 
latter fact in its turn explains why Peas and Beans are the best 
of all plant substitutes for meat, which is mostly protein. This 
relative richness of Leguminosae in nitrogenous compounds ex- 
plains also the reason underlying the ancient farming practice 



124 



The Living Plant 



of green-manuring, that is, plowing in Clover and other legumi- 
nous crops to enrich the soil. It is from these same nodules, also, 
that the Bacteria have been taken and grown for the commercial 
enrichment of the soil, as mentioned on the preceding page. 
In our consideration of these four natural methods of soil 




Fici. 1*8. The roots of several Bean plants, photoprupherl about half the natural size, 
showing the collections of wart-like nodules whirh contain the nitrifying Bacteria. 

nitrification we must not forget the artificial aid of man, who, 
for his own purposes, adds to the soil both chemical fertilizers 
and barnyard manures, with their rich supplies of nitrogenous 
and other compounds. 

Finally it is important to note that the plants, for their part, 
have a way of meeting the nitrogen scarcity of soils, viz., they 



The Various Substances Made by Plants 125 

waste none. To this end they even go so far as to remove from 
their leaves, before these are dropped, such of the nitrogenous 
and other compounds as can be used economically again. Unlike 
animals, they excrete no nitrogen, or extremely little, in either 
solid, liquid, or gaseous form, but conserve it with care and use 
it over arid over again; so that it is only released in the end by 
their decay after death. 

Class V. The Principal Poisons, or Alkaloids 

These substances are notorious as including the most violent 
plant poisons. Thus strychnine (from the Strychnos bean), 
nicotine (from the Tobacco leaves), morphine (from the milky 
juice of Poppies) are alkaloids, as is the poison, muscarine, of the 
deadliest Mushrooms. Some alkaloids, while not poisonous, 
have strong properties in other respects, such as quinine, obtained 
from the bark of the Cinchona tree and efficacious in breaking up 
fevers; caffeine, the stimulating substance in Tea leaves and Coffee 
berries; cocaine from Cocoa seeds, the well known local anaesthetic 
and fatally-alluring drug. Their meaning in the plant is uncer- 
tain, and all the more puzzling since they mostly are poisonous 
to the very plants which produce them if injected into other 
parts of their tissues. Nor is it certain just how they produce 
their poisonous effects. Alkaloids occur also in animal tissues 
as a product of the processes of fermentation and decay; they are 
called ptomaines, and are very deadly, being the real cause of 
death in bacterial diseases. Chemically the alkaloids are related 
to the amides, from which they are no doubt formed, not at all 
as a step in the formation of proteins, but as a side group. A 
typical formula is that of caffeine, C 8 H 10 2 N 4 . 

It has recently been discovered that the roots of our common 
field crops appear to excrete into the soil minute quantities of 
substances poisonous to the plants which produce them; and it is 
probable that the presence of such substances, and not the ex- 
haustion of the necessary mineral matters, is the real cause of 



The Various Substances Made by Plants 127 




in which the elements, or atoms, of the molecule, must sum up to 

more than 15,000, or even, in some cases, more than 30,000. Many 

of these substances differ little from one another in properties, 

and moreover are readily convertible one into 

another; and the facts seem to indicate that 

these elaborate forms are really multiples (or 

polymers) of some simple protein molecule, 

built up in the same manner as are starch and 

cellulose from a simple carbohydrate molecule. 

Nor is it to be supposed that all of these sub- 

stances have each a separate meaning in the 

plant, though they may have; but many of 

them no doubt are simply manifestations of 

chemical individuality in the plant, as the 

forms of starch grains are manifestations of 

physical individuality. 

Several different groups of Proteins are recognized by chemists, 

of which I shall here mention, even though in little more than the 
Homeric fashion, the more important. They 
are, Albumins, substances like white of egg, 
thinly spread through many plants: Globulins, 
which form definite grains in some seeds like 



;. M. A cell, highly 
magnified, from the 
proteinaceous layer 
just under the husk 
of Corn, showing nu- 
merous protein 
grains interspersed 
with a few starch 
(larger) grains, all 
embedded in living 
protoplasm. 




Corn (figure 39), and beautiful crystals in Castor 
\ ; * -iff B ean > Potato and some other plants (figure 40) : 
~ ---- Glutelins, typified by the familiar gluten of 

FICI.IO.-A coil, highly fl ur w hich gives the agglutinosity to dough: 

magnified, from the & & " J 

interior of a Castor Prolamins especially distinctive of the seeds of 

Bean, showing the . . . . 

crystalline protein grains i N ucleo-protems , containing phosphorus, 
sc and forming the chromosome substance of the 



what clearer by nuc i e us of cells: Phosphoproteius (called also 

treatment with re- ^ . . 

agents), em bedded in albuminates) cheese-like materials found in 

living protoplasm. , _ 

some seeds: Proteases and Peptones, very im- 
portant because they are the soluble and diffusable proteins into 
which the insoluble kinds are converted in digestion by the 



126 The Living Plant 

the sterility of some soils, which are therefore " poisoned " rather 
than " exhausted/' The composition of these substances is not 
known, except that they are complicated and perhaps nitrog- 
enous, in which case they may be found to belong with this 
group of the alkaloids. 

The reader will recall that the active properties of the alka- 
loids were somewhat foreshadowed in the nitrogenous glucosides, 
and later he will also make acquaintance with remarkably active 
properties of another kind which characterize not only the pro- 
teins entering into living protoplasm, but also the enzymes, with 
their very striking chemical powers. The common feature which 
distinguishes all of these substances in contrast with the more 
passive groups is the possession of nitrogen, which seems there- 
fore to be associated with the most active properties in plant 
substances. This fact is sufficiently curious in face of the chemical 
inertness of nitrogen, and one can fancy this element as reluctant 
to enter into combinations, restless, so to speak, while in them, 
and making disturbance in its efforts to escape to its original 
freedom. 

Class VI. The Flesh-Formers, or Proteins 

These are the most important substances made by plants, 
entering as they do into the composition of living protoplasm. 
They are more familiar in animals than in plants, for flesh is 
made up of them; but they are distributed throughout the living 
parts of all plants, either in the active protoplasm or stored as 
reserve food, especially in seeds. They are vast in number, elab- 
orate in composition, and only imperfectly known. Chemically 
they are distinguished from all of the preceding groups by con- 
taining not only the elements of the latter, but also sulphur, 
while some of them possess phosphorus too, so that their com- 
position may thus be expressed C n H n O n N n S n (P n ). Some of their 
molecules are of very great complexity; thus, there is an albumin 
with the formula C 7 2oH 1134 N 218 2 4 8 S 5 ; and there are other proteins 



i 2 8 The Living Plant 

action of peptonizing enzymes: and there are others likewise of 
rarer sort and lesser consequence. 

The mention of the presence of sulphur and phosphorus in 
proteins will lead the reader to inquire for the source of supply 
of those elements. The answer is ready. They are derived from 
soluble sulphates and phosphates absorbed from the soil by the 
roots, and are incorporated, through chemical reactions still 
imperfectly known, with the elements contained in the amides. 
All soils contain all of the sulphates that plants need, and usually 
all of the phosphates, though at times the latter are insufficient, 
and must be added as fertilizers to ensure good crops. 

Class VII. The Regulators of Metabolism, or Enzymes 

It is safe to say that the enzymes (called also ferments) are 
the most remarkable and least known, although among the most 
important, of all substances produced by plants, or by animals, 
either. They are characterized by this remarkable power, viz., 
they can cause chemical changes, each of one definite kind, in 
other substances, without themselves entering into the reaction 
or suffering any appreciable alteration. Because of this mode 
of action very small quantities of enzymes can alter chemically 
great quantities of material. Thus the enzyme diastase, which 
occurs both in the saliva of man and also in the starch-storing 
organs of plants, can convert (chemically, hydrolyze) great 
quantities of the insoluble starch by two or three steps into grape 
sugar, a soluble diffusable material; likewise the enzyme protease 
(pepsin) occurring in both plants and animals, hydrolyzes in- 
soluble indiffusable proteins into soluble diffusable peptones; 
also the enzyme lipase converts insoluble fats into soluble fatty 
acids and glycerine: cytase converts cellulose of Ivory palm and 
Date into soluble sugars; and there are many others of lesser 
prominence. It is these changes which constitute digestion, 
whether in plants or in animals. By aid of the enzymes the plant 



The Various Substances Made by Plants 129 

can not only produce and control chemical changes within its 
own body, but, by pouring them out in suitable places, can dissolve 
extraneous materials and later absorb these again for its own use. 
It is thus that insectivorous plants can digest the insects they 
capture; parasites can penetrate into the tissues of a host; and 
pollen tubes can digest their way down the solid tissues of the 
style, absorbing the digested materials for use in their own 
growth. But there are many other phases of enzyme action also; 
thus the unfermentable cane sugar is hydrolyzed (or inverted) 
to fermentable grape sugar by invertase, and grape sugar is fer- 
mented to alcohol and carbon dioxide by zymase, produced by the 
Yeast plant. And there are other cases innumerable which we 
cannot take space to consider. 

Chemically and physically we know very little about the en- 
zymes, because it has not yet been found possible to extract them 
from the protoplasm in a pure state ; and even their very existence 
would not be recognized at all were it not for their effects. It 
is not even certain that they are related to the Proteins, although 
there is indirect evidence pointing that way; nor are we sure that 
they are liquids thinly saturating the protoplasm, though this 
seems probable. Still less is it known how they produce their 
remarkable effects, although a homologous power exists in those 
inorganic substances called catalyzers. Each kind can produce 
only one chemical change, and that as a rule but a slight one, 
but the cooperation of several can cause a series of changes large 
in the end ; and it may be true that they cause most, if not indeed 
all, of the chemical processes which the living protoplasm carries 
on. They are the tools, so to speak, with which the protoplasm 
effects the chemical results it requires. Indeed to some investi- 
gators it has seemed likely that the enzymes are the principal 
material bases of heredity, and that the chromosomes of the 
nuclei, known to be conveyors of heredity, consist chiefly of col- 
lections of enzymes. Truly the importance of the enzymes is 
great, and their further study in the near future is likely to throw 



130 The Living Plant 

much light upon some of the most fundamental problems of Bi- 
ology. 

Class VIII. Living Protoplasm 

This substance is of such importance and complexity as to re- 
quire for its treatment, a separate chapter, which follows. It 
need only be said in this connection that so far as chemical 
analysis has been able to penetrate into the mysteries of living 
protoplasm, it appears to be merely a very complicated mixture 
of proteins with many simpler substances. Here for example 
is a list of the substances which have been recognized in a chemical 
analysis of the protoplasm of one of the lower plants; 

Water, Pepsin and Myosin, Vitellin, Plastin, Guanin, Xanlhin, 
Sarkin, Ammonic carbonate, Asparagin and other amides, Pepton 
and Peptonoid, Lecithin, Glycogen, Aethalium sugar, Calcic com- 
pounds of higher fatty adds, Calcic formate, Calcic acetate, Calcic 
carbonate, Sodic chloride, Hydropotassic phosphate, Iron phosphate, 
Ajnmonio-magnesic phosphate, Tricalcic phosphate, Calcic oxalate, 
Cholesterin, Fatty acids extracted by ether, Resinous matter, Glycerin, 
coloring matter, etc., Undetermined matters. 

In this list, which I give in order to illustrate the chemical 
complexity of protoplasm, all of the constituents are well-known 
substances, no one of which has any of the properties of life, 
unless such a substance lies hidden in the trifling amount of "Un- 
determined matters" ; nor has any chemist yet been able to identify 
any distinctive living substance, any of that protoplasm par 
excellence which we are logically bound to believe must exist. 
But the further consideration of this subject belongs with the 
next chapter. 

Such are the groups of substances which plants build upon the 
foundation laid by the photosynthate. We may summarize their 
relationship in a diagrammatic manner, after the analogy of a tree 
of ascent, as shown herewith. 



The Various Substances Made by Plants 



-Alkaloids 



Amities 



Living Protoplasm It may perhaps have occurred to the 

reader ere this to inquire what proportion 
of the original basal photosynthate is used in 
Proteins the construction of each of these classes of 

substances. The question is a fair one but 
difficult to answer, partly because the pro- 
portions would be so different with the vari- 
ous kinds of plants, and partly because we 
have so few data for making calculations. 
However, it is possible to make a generaliza- 
tion for plants as a whole, and this has been 
(Photosynthate) done in the table below^, which, although lit- 

tle more than a guess, has yet some value. For simplicity I have 
reduced the table to the kinds of known and visible substances, 
grouping together the others as " special substances " ; and inci- 
dentally I have added the ultimate fate of the various groups. 



^Oils-Resins 



Carbohydrates 



<N 



<M 


10" 


O 


iO 


1 


9 


o 


CD 




w 


O 


q 


a 


ft 


ft 


a 


ft 


w 


si 

8 


6 





O 



<N 

O 


6 
u 


5 

o 


o 


o 










02 


8 














o 








c? 


c3 










O 


O 


> 








T5 


T3 


c3 










faC 



0) 












8 

s 
zS 



!f!f 



O Hg 



T3 







a g 



P 



-2 -s a 

o 5 g 






3 

O 



c3 

H 



a 

8 



132 




The Various Substances Made by Plants 133 

This table brings out clearly once more that most fundamental 
of facts about the physical constitution of living things, that their 
substance is all derived originally from carbon dioxide and water, 
with a few minor additions, and is 
all returned in the end back to the 
same source, undergoing en route 
transformations of substance and 
energy which constitute the princi- 
pal visible phenomena of life. The 
organism is made up of a little 
of those substances temporarily 
withdrawn from the general cir- 
culation of nature and interacting FIG. 4 1.- A coil, highly magnified, from 

Vigorously with One another Under Benonia, showing a mass of crys- 
^ J tals composed of caleic oxalato, lying 

the Stimulus Of external forces, within the cell-cavity around which 

can be seen the living protoplasm. 
principally the SUn. Organisms (Copied from a wall-chart by L. 

are, as it were, little whirlpools in Kny ' 

the general circulation of matter and energy. And I cannot for- 
bear to attempt to illuminate this matter somewhat further by 
aid of one of my favorite diagrams, which is presented herewith 
(figure 42). 

There is yet one other group of substances made by plants, 
very different, however, in kind from those already described. 
In the tissues of all plants the microscope reveals mineral matters, 
sometimes in great abundance and crystallized in very beautiful 
forms, of which our illustration (figure 41) gives some, though an 
inadequate idea. A few are probably useless minerals absorbed by 
the roots along with the useful kinds presently to be noted, but the 
great majority are by-products of useful chemical reactions. Thus, 
the commonest of the crystals is oxalate of lime, which is formed 
from oxalic acid, probably a by-product in the manufacture of 
proteins. These crystalline matters are obviously of no use, but 
are waste materials. In the absence of a regular excretory system 
such as animals possess, the plant has no resource except to store 



\ \ \ \ \ \ 




Fio. 42. A diagram illustrative of the relation of plant and animal life to the circulation 
of the principal substances of nature. 



134 



The Various Substances Made by Plants 135 



them up in out of the way places, though they may ultimately 
be partially removed by the fall of the leaves and the bark. 

There remains one other important phase of our subject. It 
concerns the indispensability of certain elements for healthy 
metabolism, although they do not enter into the composition 
of any of the substance manufactured. Everybody knows that 




Lacks, all potas- 
sium 



cal- 
cium 



nitro- phos- mag- 
gen phorus nesium 



iron nothing 



FIG. 43. Illustration of the method and results of water culture. The plants are Corn, 
all started at the same time. (Copied from a wall-chart by Errera and Laurent.) 

potash (potassium) is thus indispensable, to such a degree that 
it must often be added as a fertilizer to soils; but its symbol (K) 
is not found in any of the formulae cited in this chapter. The same 
is true of the elements calcium, magnesium and iron, and probably 
sodium and chlorine, all of which are indispensable to the healthy 
growth of most or all plants, but none of which enter into the 
composition of the most important plant substances. Naturally a 



136 



The Living Plant 



great many attempts have been made to determine the exact 
function of each substance, and why it is essential. The reader 
will be interested in the principal method used to this end. It 
depends on the fact that there are plants, and many, which will 

grow through their whole 
cycle from seed to seed in 
water, without any contact 
with soil, if only the needful 
minerals be contained in the 
water. This method is called 
water culture, and the prac- 
tical arrangements therefor 
are well shown in the accom- 
panying figure (figure 43), 
while a product of the 
method, produced in my 
own laboratory, is shown by 
figure 44. Now, by growing 
one plant in water contain- 
ing all of the necessary min- 
erals except one, side by side 
with another plant grown in 
water containing all of the 
needful minerals, it is possi- 
The screen is ruled b ] e to observe what effect 

the absence of this one sub- 
stance produces, and hence to infer what its use to the plant 
must be. The general results of an experiment of this kind 
are well shown in figure 43. In this way we have found that 
potassium is necessary to the formation of the photosynthate, 
calcium to its transfer through the plant, and iron to the 
formation of chlorophyll (into the composition of which, how- 
ever, it possibly enters); but further than this, and as to the 
other materials, our knowledge is most vague and unsatisfac- 




FIG. 44. Corn 

in a common tumbler. 
in centimeters. 



The Various Substances Made by Plants 137 

tory. It seems quite plain, however, that the role of these 
elements lies in services incidentally necessary to the greater 
processes, such as aiding in chemical steps, neutralizing poison- 
ous excretions, and so forth. They are like the servants at a 
party; they are indispensable to its success, but their names do 
not appear in the list of those present. But our ignorance on 
these matters, and upon so many other phases of our subject of 
metabolism, is only acting as a spur to the efforts of many de- 
voted workers, who, in laboratories all over the world, are attack- 
ing these problems with the full determination to solve them. 
The methods of science are slow, but they are irresistible; and 
the solution of the problems is only a matter of time. 




CHAPTER VI 

THE SUBSTANCE WHICH IS ALIVE IN PLANTS, AND ITS 
MANY REMARKABLE QUALITIES 

Protoplasm 

ALREADY more than once in this book the reader has 
met with a mention of protoplasm, the living sub- 
stance of plants. Besides, almost everyone has some 
knowledge about it, or thinks that he has, though much 
of the current information is a very long way from the truth. 
There are even some persons who believe that protoplasm is an ab- 
stract conception evolved by the mind of man to help explain phe- 
nomena otherwise incomprehensible; while a few seem to cherish 
the idea that it is one of the many inventions sought out by science 
for undermining the faith. Yet protoplasm is not any of these 
notions, but a real material which can be seen, handled, and sub- 
jected to experiment. The reader will wish to know the facts 
about this most important of substances, and here is the suitable 
place to consider them. 

It is nowadays an educational axiom that a good understanding 
of any scientific subject is possible only through personal contact 
arid experience with the matter in question. A great many people 
do not comprehend this necessity, and believe that well-written 
and fully-illustrated books are a sufficient, if not actually a supe- 
rior, substitute for the laborious and time-consuming methods 
of the field or the laboratory. When the reader meets with this 
error he can refute it effectually by asking the objector whether 
he considers that guide-books, even the best written and most 

138 



The Substance Which Is Alive in Plants 139 



profusely illustrated, are a satisfactory substitute for foreign 
travel. The case is still stronger with scientific facts and phenom- 
ena, for these are mostly of a sort even more foreign to the stu- 
dent^ previous experience than are the sights and impressions 
of distant lands. All this is quite true of the subject before us, 
and if the reader would really understand the substance Proto- 
plasm he must take steps to see it for himself, even if he has to 
trouble some friend, his physician, or the nearest botanical ex- 
pert, for the use of a microscope. 



I 






Fro. 15. Typical colls, in optical section highly magnified, of hairs from Spiderwort, 
Gloxinia, and Squash, respectively, showing as accurately as the author can represent 
it by pencil, the appearance of their gray-granular threads and lining of living pro- 
toplasm. 

If, now, the reader will carefully remove some of the younger 
of the hairs which are so prominent in the flowers of the common 
Spiderwort of the gardens, (or the closely-related Wandering Jew 
of greenhouses), or some of the hairs on the young leaves or stems 
of Squash, or Gloxinia, (or even of " Geranium"), will place them 
on a glass slide in a drop of water, cover them with a thin glass, 
and then examine them with the microscope, he will see before him 
living protoplasm, the most remarkable of all natural substances. 
These particular objects display an appearance represented in the 
accompanying pictures, (figure 45) ; and they have an advantage 



140 The Living Plant 

over others which might be chosen in this, that while compara- 
tively easy to obtain, their protoplasm exhibits a streaming 
motion, which, though often slow and difficult at first to detect, 
nevertheless when seen forms a valuable proof of its living condi- 
tion. The rather inconspicuous grayish-granular, translucent, 
semi-fluid appearance here presented inside of the cells is repre- 
sentative of the aspect of protoplasm in general. The granular 
look is due largely to the presence of food granules, which in 
some cases are absent, leaving the protoplasm so nearly trans- 
parent that it can hardly be seen at all unless stained by some dye, 
while in other cases the granules are so plenty as to give the proto- 
plasm an appearance of solidity. Moreover, as these granules 
consist largely of protein which has a slight yellowish color, they 
give to protoplasm in dense masses a distinctly yellowish or 
brownish-yellow tinge; and this is the cause of the yellow color 
which shows so plainly through the tips of white roots, and of 
the brownish-yellow of the interior of young ovules. In the hairs 
supposed to be lying before the reader, the protoplasm is obviously 
soft enough to flow freely, though it is not wholly a fluid; and it 
is known to possess about the consistency of a soft jelly. Indeed, 
if one were to imagine an uncolored jelly, somewhat too soft to 
retain the form of its mold and all clouded instead of quite clear, 
in other words just the kind of jelly that the thrifty house- 
keeper doth most despise, he would have a very good idea of the 
protoplasm of these hairs. In some plant tissues the substance 
is still softer and almost a liquid; in others it is firmer, to such a 
degree that in seeds it becomes tough and hard as horn, though 
never approaching the hardness of ivory, as a prominent diction- 
ary says that it does. The visible streaming of the protoplasm 
in these hairs, however, is not typical, for while in some kinds 
the streaming is even more active, generally it is very much slower, 
and commonly is imperceptible; so that the reader must not 
allow the motion to become too prominent a feature of his vis- 
ualization of plant protoplasm. A white-granular, slow-moving 



The Substance Which Is Alive in Plants 141 

jelly; that is what protoplasm looks like, and that is precisely 
what it is.* 

While protoplasm for the most part can be observed in plants 
only by aid of the microscope, there are cases in which it occurs 
in masses sufficiently large to be studied by the unaided eye, and 
to be taken in the hand. Everybody has seen those soft, whitish, 
slimy masses which are flattened against decaying wood in damp 
dark places, such as the rotten underpinning of old buildings, 
in cellars and dark greenhouses, or on old shaded tan-bark, 
whence they are known as "Flowers of Tan." These are called, 
scientifically, Slime-molds, and they are practically pure naked 
protoplasm, the accessibility of which has made these low plants 
very favorite objects for protoplasmic studies. 

Such is the appearance of living plant protoplasm as seen 
by the eye or through an ordinary microscope; and try as one 
will, he can see little more. The supreme importance of proto- 
plasm among earthly substances has of course acted as a stimulus 
to the most thorough researches into its structure; and all the 
highest powers of the microscope, and all the most refined de- 
vices and methods known to microscopical science, have been 
brought to bear upon it. Yet these efforts have yielded little 
additional knowledge, and even that little has been left involved 
in uncertainty and controversy. We do not even know what tex- 
ture the protoplasmic substance possesses. Some investigators 
have concluded that such protoplasm as the reader has seen 
streaming in plant-hairs is a loose network of fine elastic fibers, 

* The streaming of Protoplasm is thus vividly visualized, though with some ex- 
aggeration natural at that time, by Huxley, "Currents similar to those of the hairs 
of the nettle have been observed in a great multitude of very different plants, and 
weighty authorities have suggested that they probably occur, in more or less per- 
fection, in all young vegetable cells. If such be the case, the wonderful noonday 
silence of a tropical forest is, after all, due only to the dulness of our hearing; and 
could our ears catch the murmur of these tiny Maelstroms, as they whirl in the in- 
numerable myriads of living cells which constitute each tree, we should be stunned, 
as with the roar of a great city." The Physical Basis of Life in his Collected Essays, 
New York, I, 130. 



142 The Living Plant 

holding liquids in its meshes as a sponge might do, a view more 
prevalent formerly than now, even though it is sustained by the 
appearance of the substance when killed and colored by dyes. 
Others consider that protoplasm, aside from certain solid gran- 
ules, is chiefly an emulsion of various liquids, which rest suspended 
as tiny globes in a matrix of fluid ground substance, very much as 
the tiny globules of oils remain suspended in water after violent 
shaking of a mixture. And the advocates of this view, now in the 
ascendant, have supported it by constructing, out of ordinary 
chemicals, certain emulsions or foams, which show striking sim- 
ilarities to living protoplasm not only in appearance, but in move- 
ments, though they are, however, far enough removed from 
protoplasm in all other respects. And a third view tries to har- 
monize the two others by supposing that some protoplasm has 
one structure and some the other. In one part only does proto- 
plasm display a definite structure, and that is in the nucleus dur- 
ing reproduction, a matter we shall presently consider. 

It may seem to the reader remarkable that I do not attempt to 
illustrate so important a subject more fully by pictures. But 
protoplasm in fact, because of the lack of clear definition in its 
structure, is most difficult to represent well in any kind of pic- 
ture. Indeed, hardly any two persons represent it alike, as 
follows naturally enough from the fact that hardly any two per- 
sons see it alike. In various figures in this book, however, I have 
tried incidentally to give some, even though rather a conventional, 
idea of its appearance, and to these figures (figures 33, 34, 39, 40, 
41 , 45) the reader will now find it worth while to refer. And I shall 
at this point, add one more, and one of the best, in which the great 
botanist Sachs has tried to represent it as if projected against 
a black background, (figure 46). 

We come now to the important matter of the chemical com- 
position of protoplasm, from which, in view of its many remark- 
able powers, we naturally anticipate something of very unusual 
interest. The most striking of the chemical facts about it, as the 



The Substance Which Is Alive in Plants 143 

chapter on Metabolism further illustrates, is this, that proto- 
plasm, despite its aspect of simplicity, is not a single substance, 
but a very heterogeneous mixture of many different substances 
of diverse grades of complexity, from the simplest of mineral 
salts up to the most complicated of pro- 
teins. None of these substances, how- 
ever, are of themselves alive, nor has 
chemical analysis yet succeeded in lo- 
cating any distinctively living constit- 
uent, any protoplasm par excellence, 
although we are logically bound to be- 
lieve that some such substance must 
exist as a seat for the distinctive prop- 
erties of life. Protoplasm, therefore, is 
probably composed chemically of two 
classes of materials; first, a very small 
amount of a distinctively living constit- 
uent, not yet identified, but consisting, 
in the fibers, or else the ground substance 
of its physical texture; and second, a very 
large amount of various non-living sub- 
stances, nutritive and other, which are 
under the control of the living constit- 
uent. 

There are, however, some further 
chemical facts about protoplasm which FlG> 46 ._ The prot opiasm of a 
go a little way towards explaining its hair ceiiof a Gourd, projected 

^ JT o against a black background. 

various powers. Thus, a part of its con- (Reduced from Sachs' Leo 
stituents (in general the most compli- 
cated) are very unstable, or, chemically stated, labile, and 
change their composition under slight provocation whether from 
without or within. Such changes are accompanied, like all 
others of a chemical nature, by transformations of energy, either 
release or absorption. And these in turn cause other changes, 




144 The Living Plant 

and these yet others, in an almost endless succession. Thus liv- 
ing protoplasm, complex and unstable in its constituents, and 
acted upon constantly by diverse forces both from without and 
within, is a constantly seething mass of energy-and-material 
changes; and it is such changes which constitute the visible 
phenomena of life. But, and here is the crux of the matter, 
these changes are not hap-hazard and aimless, but on the contrary 
proceed in a definite and orderly sequence, resulting in the forma- 
tion of definite structures and the performance of definite actions 
time after time and generation after generation; and it is this 
orderliness, this definite procession of physical and chemical 
processes, rather than anything in the processes themselves, 
which is the most distinctive characteristic of life. The failure 
of the regulatory power breaks the circuit of the processes, and 
leaves the protoplasm a helpless mass of matter all ready for de- 
cay; and this failure we name death. Life thus consists of two 
elements, first, material and energy changes, that is, purely physi- 
cal and chemical processes, whose general nature we can under- 
stand, and which are seated in the various substances that chem- 
ists have identified in the protoplasm, and second, a regulatory 
power which directs and makes use of those processes but whose 
nature and location is still quite unknown. Perhaps the nature 
of this regulatory power is incomprehensible, or unknowable, in 
our present philosophies, though as to that, science never admits 
that anything is unknowable, but works ever under the assump- 
tion that everything can be known if we but refine sufficiently our 
methods of investigation. 

There is one other feature of the chemistry of protoplasm 
which may have some importance in explaining its powers. In a 
general way it seems true that the protoplasm of the higher and 
more elaborate plants and animals is more complicated chemi- 
cally, or at all events produces a greater number of complicated 
substances (proteins especially), than the lower. This suggests 
that each of the special physiological features successively ac- 



The Substance Which Is Alive in Plants 145 

quired by plants and animals in the course of their evolution 
has its seat in a special chemical constituent of the protoplasm. 
On this view, evolution, physiologically considered, depends upon 
chemical experimentation, so to speak, in the protoplasm, and 
follows step by step on the successful formation of new chemical 
compounds. But let the reader beware of accepting this sug- 
gestion as knowledge; it is merely a speculation, but one of those 
which, in science, it is legitimate to throw out ahead as a tem- 
porary guide to further investigation. 

In common with all other substances in Nature, protoplasm 
thus possesses its physical and chemical properties. But in ad- 
dition it possesses another set not found in other substance; 
and thereupon depend its powers to do the remarkable things 
that it does. These may be termed its physiological or vital 
properties, which are as follows; the property of metabolism, or 
power of causing orderly chemical changes within itself, including 
photosynthesis and respiration, and the other changes recorded 
in our chapter devoted to that particular subject: the property 
of conduction, or power to transport substances in definite paths 
through itself, including absorption, transfer, and excretion: the 
property of growth, or power to incorporate new material and to 
increase in size at special places: the property of division, or power 
to separate portions of its own substance, the basis of reproduc- 
tion: the property of mobility, or power to cause definite move- 
ments of its own substance, the basis of protoplasmic streaming 
and locomotion: the property of irritability (sensitivity), or power 
to respond advantageously to various stimuli. This enumeration 
of the physiological properties of protoplasm reads like the table 
of contents of a book on physiology, and it ought to, because 
physiology is nothing else than a study of the properties of proto- 
plasm. And here is a point of importance. Just as the physical 
properties of any substance are believed to reside in certain ulti- 
mate structural units, which are the smallest portions into which 
that substance can be divided and still retain those properties, 



146 The Living Plant 

and which units in this case are the molecules, and just as the 
chemical properties are supposed to reside in their ultimate units, 
in this case the atoms, so the vital properties must be supposed 
to reside in some kind of units distinctively their own. These 
units, obviously, must be larger than the molecules and made up 
of organized aggregates thereof. They have been called by various 
names, notably plasomen, (in the singular, plasom), and arc 
probably identical with the micellae of which we shall have much 
to say in the chapter on Absorption. All substances are made up 
of atoms and molecules; protoplasm alone is made up of atoms, 
molecules and plasomen. And the reader will observe, by the 
way, that the very conception of the plasom involves the idea of a 
distinctive protoplasmic main substance, and constitutes indeed, 
an additional reason for believing in the existence thereof. 

As one views the various physical features of protoplasm, and 
thinks of the remarkable things it can do, he cannot but wonder 
at the discrepancy between its aspect and its accomplishments. 
For protoplasm is one of the most insignificant in appearance 
of all substances, yet secures the most wonderful of all results. 
For has it not built the whole plant and animal world, culminating 
in man with his powers of thought? Yet this discrepancy be- 
tween promise and performance is not without parallel in our 
human experience. If some stranger from far away space, where 
all things are differently done, were to visit this earth and be 
shown the multifarious works of man's hands, and were after- 
wards to have man pointed out as their maker, he would doubtless 
exclaim in astonishment; "How can a creature so small build 
these cloud-cleaving towers a hundred times loftier than himself, 
or these huge leviathans of steamships ten thousand times bigger 
than he: or how can a thing so weak raise pyramids so ponderously 
colossal: or one so slow of foot drive such fleet-flying engines: 
or one with hands so soft bore tunnels through miles of solid 
rock?" Man gives no suggestion in his appearance of the nature 
of the power whereby he does these things, for that lies not in 



The Substance Which Is Alive in Plants 147 

his visible body but his invisible mind, which enables him to 
plan and make use of tools, and harness the restless forces of 
nature. So, we can only suppose that the physically-insignificant 
protoplasm accomplishes its results by some analogous power. 
Indeed, I venture for my part to believe that all protoplasm can 
think, not mind-thought it is true, for that appears to belong 
only to man, but body-thought of which the mind is unconscious. 
Or the matter may better be stated in this way, that man's thought 
is but the conscious form of a principle which exists unconsciously 
through all living substance. All protoplasm thinks, but only 
the portion thereof in man's brain is aware that it thinks. How- 
ever this may be, there is one thing that is plain; man's is not 
the only protoplasm which makes use of tools, and compels the 
forces of nature to do its work, in evidence whereof let the reader 
observe, for example, what is said in this book about enzymes, 
and the dissemination of seeds. 

We must here turn back for a moment to the chemistry of 
protoplasm in order to notice a matter important to an under- 
standing of the relations of the substance to the external world. 
The chemical complexity and instability of protoplasm render 
it extremely sensitive to the effects of external influences, which 
act upon it in three different ways. First, if strong enough, they 
act upon it forcibly, precisely as upon any other substance of 
comparable sort, and quite without reference to whether it is 
living or not. Thus, heat burns it; pressure crushes it; and some 
chemicals dissolve it. Second, the forces when too weak to exert 
any forcible effects, can yet act inductively to promote, or to 
check, some of the processes in progress in the complicated chemi- 
cal laboratory which the living protoplasm actually is, and thereby 
may produce a profound effect upon the behavior of the plant 
as a whole. Thus heat, in a degree far too low to injure the 
protoplasm, promotes the activity of those physical and chemical 
reactions which underlie the streaming, nutrition, growth and 
other activities of protoplasm; and this explains why protoplasm 



148 The Living Plant 

streams faster, and plants grow better, in warmth than in cold. 
Light acts analogously on the cell-contents, and one of the results 
is the brilliant redness of autumn coloration. In some cases the 
external factors, especially some chemical substances, act repress- 
ively on the processes, which explains the action of anaesthetics. 
Third, the factors, when far too weak to exert even an inductive 
effect can act in a far more remarkable and consequential manner, 
for they can then serve as guides, or stimuli, in response to which 
the protoplasm can send its parts into positions found by past 
experience to be best for the performance of its functions or avoid- 
ance of dangers. Thus, light far too weak to be directly useful 
or injurious to the plant yet serves as a guide whereby stems can 
grow towards it, leaves across it, and roots away from it, those 
positions being the most advantageous for the performance of 
their particular functions. And innumerable other cases of this 
kind are known, of such interest and importance, however, that 
they must receive a chapter all to themselves under their proper 
physiological name of Irritability. It is enough for our purpose 
at present to make clear the existence of the three-kind relation 
between protoplasmic activity and the external world. 

One does not go far with his studies upon protoplasm before 
he begins to take thought of its origin. In one way the prob- 
lem is simple enough, for all of the protoplasm familiar to us 
originates obviously in only one way, by growth and division 
from other protoplasm through reproduction. It is not so long 
since even scientific men held the contrary belief, still widely 
persistent among uneducated folk, that low forms of life could 
originate anew in slime or other fermentable masses; but later 
experimental studies, chiefly led by the great Frenchman Pasteur, 
have shown that in all such cases living germs are present, while 
if precautions are taken to kill all germs by heat or suitable 
poisons, then no life appears. Every known case of apparent 
spontaneous generation having thus been investigated and dis- 
proved, we infer that probably it does not now occur in our 



The Substance Which Is Alive in Plants 149 

world, and that all protoplasm nowadays originates from pre- 
cxistent protoplasm through reproduction. This much is easy. 
But when we try to trace back the continuously-reproducing 
chain to its very first origin in time, we come soon to the limits 
of our knowledge. Some philosophers have suggested that the 
germs of life were first brought to the earth in meteorites from 
other planets; but this merely sets back the difficulty one stage 
and does not remove it. Another explanation, which seems to be 
that most commonly assumed by scientific men, places its origin 
in spontaneous generation at some time in the earth's history 
when the favorable combination of material and energy happened 
to occur. Obviously, such a combination ought to be repeat able 
experimentally; and it is upon this assumption that many learned 
men, from astrologers of old to physiologists now with us, have 
sought, though in vain, to make protoplasm anew in the flasks 
of their laboratories. There is, however, a third explanation which 
I have already suggested in an earlier chapter, namely, that the 
protoplasm known to us did not originate in its present form, 
but is evolved or descended from a simpler substance adapted 
chemically to the higher (or lower) temperatures which formerly 
prevailed on the earth, while that substance in turn was evolved 
from a still simpler, and so on backwards to a beginning cotempo- 
raneous with that of inorganic matter itself. This view I hold 
to be the most reasonable and probable. 

But, after all, the most impressive and important thing about 
protoplasm is its power to build those great and elaborate struc- 
tures which we call plants and animals. For, structurally con- 
sidered, a plant or an animal is nothing other than a mass of 
soft protoplasm which climbs aloft and reaches outward into the 
form of the plant or the animal, building itself meantime a skele- 
ton for the support of its helplessly-weak substance. Now, in 
building these organisms, the protoplasm never exhibits the char- 
acter of a continuous and homogeneous mass, but always sepa- 
rates partially into tiny structural units called cells, which are 



150 The Living Plant 

mostly too small to be seen by the naked eye, but which appear 
prominently in every magnified view of any part of any animal 
or plant, as witness, for example, figures 2, 53, 73, 141, in this book. 
We must therefore consider with some care the construction of 
these cells, a subject of the foremost importance in Biology. 
The hairs earlier studied are fairly typical cells except that 

they are partially isolated from their 
neighbors instead of deeply em- 
bedded among them, and are elon- 
7 W * , gated rather than rounded. If one 

cytoplasm 

observes an example of these hairs, 
e. g., that of the Squash (figure 47), 
- nuHeoTus h * s likely to notice first the clear- 
- - piastid cut containing wall, inside of which 
-- sap-cavity comes a complete lining of soft gray- 
granular protoplasm, very likely in 
^ slow streaming motion, with threads 

FI.I. 47. An optical section, highly f t} extending across the cell 

magnified, through a cell of tho " 

Squash, HhowiiiR all the parts of a at VaHoUS angles. ThlS Soft proto- 
typical cell. 11 i , i o 

plasm is called cytoplasm, some- 
where within it, though not carried in its streaming, lies a denser, 
rounded granular structure, also living protoplasm, the nucleus, 
which often exhibits a round mass within itself, the niiclcolus. 
In the cytoplasm lie also certain scattered granules (not especially 
distinct, however, in these hairs), which are larger than food gran- 
ules and otherwise unlike them; these too, are living protoplasm, 
and are called pla-stids. Finally, within the cytoplasm appear 
large open spaces, various in size and number but commonly 
merged to a single very large one in old cells; though apparently 
empty, they really are filled with a watery sap and therefore are 
known as sap-cavities. These parts, wall, cytoplasm, nucleus, 
plastids, sap-cavities, are the prominent parts of typical plant 
cells, and the great majority of cells possess them all. We can 
accordingly construct a conventionalized cell showing these 



The Substance Which Is Alive in Plants 151 

parts in their natural relations and fully-developed condition; 
and such a cell is represented herewith (figure 48). 

We should now examine a bit further these parts of the cell 
and their meaning. 

The wall is composed of a firm-elastic transparent substance 
called cellulose, whose chemistry is treated in the chapter on 
Metabolism. It is built by the cytoplasm, which, in suitable 
places, is supposed to lay down within itself tiny masses (bricks, 
us it were) called micellae, of cellulose, and continues to add to 



wall 

plastic! 
SVy.. sap-cavity 




nucleus 

micleolus 

'- cytoplasm 



FIG. 48. An optical section through a conventionalized complete plant cell. 

their number until they accumulate to nearly a solid mass. I 
say "nearly," because apparently there always are left between 
these micella) thin sheets of protoplasm, like the mortar between 
bricks, so long as the cells are alive, though they are withdrawn 
when the cell has reached full maturity. It is these thin sheets 
of cytoplasm, too thin to be visible even to the strongest micro- 
scope, which keep the wall alive, as it were, so that it can become 
enlarged, split, chemically changed, absorbed in places, and in 
other ways altered, a good while after its formation. But except 
for such subsequent alterations, the walls of contiguous cells re- 



152 The Living Plant 

main parts of one continuous mass. As to the function of the wall, 
that is perfectly obvious, it is the skeleton of the cell, the me- 
chanical support for the gelatinous cytoplasm, which has not 
enough firmness of texture to raise itself unaided an inch from the 
ground. It is interesting to let the imagination picture what 
would happen to the loftiest and stateliest tree, if, by some subtle 
chemical magic the cell-walls could be suddenly re-converted back 
to the gases from which they were made; the protoplasm would 
simply collapse to the ground as a shower of slime. 

The reader at this point will observe how different in principle 
is the construction of the skeleton in plants as compared with 
animals. In animals, in conformity with the much higher de- 
gree of division of labor in their parts, certain cells are set aside 
to build the skeleton for the entire individual, either a deeply- 
buried bony skeleton as in man, or a surface skeleton of lime 
or horn as in crabs and insects; while all of the remainder of their 
cells are without hard walls and devoted to other functions. 
In plants, however, every individual cell has a wall around it- 
self, and the collective mass of these walls makes up the skeleton 
of the plant. Such a mass of cell-walls, however, by no means 
represents, though one might naturally think so, a lot of origi- 
nally separate walls fused together. Observation of growing parts 
always shows (figure 101) that the new walls formed between 
dividing cells are thrown across the protoplasm as single solid 
structures, which may or may not in time become split and 
divided between the two cells. Thus the cell-wall system of a 
plant is one single mass from the beginning, just as is the wall 
mass of a building; and the protoplasm lives in cavities therein, 
precisely as people live in the rooms of a house they have built. 
The reason for the difference in the method of skeleton building 
by animals and plants is plain enough upon reflection. The 
method of animals permits jointing and muscular movement, 
as it must in order to allow the most fundamental of all animal 
activities, locomotion in search of food; the method of plants 



The Substance Which Is Alive in Plants 153 

permits only a fixed position, which, however, is sufficient, since 
the materials for making their food are brought to them in the 
general circulation of nature. And these conclusions are all the 
more confirmed by the seeming exceptions, for some plants swim 
or creep freely about (e. g., swimming spores of Algre and Slime 
Molds) in a very animal-like manner; but in these cases they lack 
the firm cellulose wall distinctive of plants. But although the 
skeletons of animals and plants differ, their protoplasm does not, 
for in all essentials the protoplasm of plants and animals is alike. 
This brief account of the plant skeleton has touched incidentally 
on a matter which must now receive some further attention. As 
the student soon learns when he studies many cells with his 
microscope, they differ immensely in shape and in the thickness 
and composition of their walls, to such a degree indeed as to 
make them apparently too complex for analysis. Yet here, as 
elsewhere, further study gradually crystallizes out the essentials, 
when it appears that after all only a few ground forms exist, and 
then only in correlation with definite functions or influences; 
while all of the others are simply variations and combinations of 
these. As to the shapes of cells, the simplest of all, and the one 
to which all others tend to revert, is the sphere, that being the 
mathematical form in which the most contents can be comprised 
within the least wall. This shape, with the wall a spherical shell, 
is actually realized in those cells which float freely in water or 
air, as do the spores of many Algae and Molds, and some pollen 
grains; and this shape may become elongated to ellipsoid and ovoid 
forms under particular conditions (figure 49, 94, 108) . Where such 
cells occur inside the tissues of plants, however, and hence are 
hard pressed by their numerous neighbors, the spherical shape 
becomes necessarily modified to many-sided (polyhedral) or 
faceted; and this shape is approximately realized in many stor- 
age tissues of plants, where it comes measurably near to that 
twelve-faced shape which always results when equal-sized spheres 
are forced together by pressure (figure 49, 72). There is also some 



154 



The Living Plant 






O 








FIG. 49. Generalized drawings of optical sections through the principal forms of plant 
cells, all of which are derivable by differential growth from the spherical form in the 
center. 

approach to this shape in the green cells of leaves, (figure 2), al- 
though here a modification is introduced by the need for con- 
centrating the chlorophyll grains towards the best-lighted sur- 
face, for which a cylindrical shape is the best (Plate I, #), or else 



The Substance Which Is Alive in Plants 155 

by the need for the presence of very large air spaces, for which a 
branching, or stellate form is most suitable. The polyhedral 
shape due to mutual pressures, in conjunction with the formation 
of new walls as plates thrown across cells from one wall to the 
other, results in the formation of cubical cells in growing points 
(figures 49, 53, 139 CD), or elongations thereof to four-sided 
prisms, as in the cambium cells, which form the growth zone 
between the bark and the wood in most trees (figure 139 J3). In 
other cases the cells become flattened to tabular shapes, as in 
epidermis and cork (figures 2, 49) ; where the function of those 
colls as the protective skin of the plant obviously requires such a 
shape. Again, the spherical or polyhedral shape becomes elon- 
gated to a cylindrical or prismatic form where the function re- 
quires much length, as it does in the conduction of liquids through 
the plant; and it is a line of such cylindrical cells, thrown into 
a tube by absorption of the intermediate walls, which constitutes 
the water-carrying ducts, (figures 49, 53, 54 C, 72) while the food- 
carrying sieve-tubes are made in analogous manner (figure 72). 
Or, the elongation takes place at two opposite points, result- 
ing in a spindle or fiber form, which is developed wherever tensile 
strength for resistance to strains is required (figures 49, 50 d). Fi- 
nally, through the intermediation of a more active growth at several 
points, the spherical or polyhedral shape becomes modified to a 
branching, or even a star-shaped form; and this occurs in the 
spongy cells of green leaves as a means of providing generous 
inter-cellular air spaces, (figure 2, and B of Plate I): in some 
Rushes as a part of their very flexible pith (figure 49) : and in 
certain excretion cells of Water-plants as a means of providing 
more wall for the deposition of waste crystals. Thus these few 
ground forms, the fundamental sphere, with its lines of modifi- 
cation, shown by figure 49, viz., ellipsoid-ovoid, polyhedral, 
tabular, cylindrical-tubular, spindle-fibroid, and branched-stellate, 
represent the mathematical possibilities upon which the cells can 
play, but by which they are also bound in their adaptations to 



156 



The Living Plant 



their various functions; and although innumerable forms occur 
not directly referable to any of these types, they are never- 
theless only modifications and combinations thereof. 

The cell-wall, however, is modifiable not only in shape, but also 
in thickness. Ordinarily very thin, it can become thickened to 
any degree required by function, even to the almost total ob- 
literation of the cell cavity, as happens in some fibers (figure 50, rf), 
where the need for additional strength is perfectly plain : in cells 







FIG. 50. Various methods of adaptive thickening of cell-walls; further particulars in text. 
(All copied from von Mohl's classical work on the Plant Cell, 1851.) 

devoted to the protection of something, notably in the shell of a 
nut (figure 50, a) : and in cases where the formation of a thickened 
wall is a means of storing additional food, as in the Ivory Palm and 
Date (figure 36). A similar thickening is used also as a pro- 
tection to the resting spores of Molds, Yeasts and disease germs, 
which thereby are so completely protected against all hostile 
outside influences that they can float uninjured for months in 
the air, and germinate finally in the most unexpected and least 
desired of places. In some cases the thickening is not at all uni- 
form, but takes the form of rings and spirals, as in young ducts 
which they help to keep open while the walls are still very flex- 
ible (figure 50, b) ; or it makes an elaborate fretwork of strength- 
ening ridges surrounding thin areas easily pervious to water, 
as in older ducts (figure 50, c) ; or it occurs upon one wall only, as 



The Substance Which Is Alive in Plants 157 

is frequently the case with protective epidermal cells (figure 50, e) ; 
or it affects only the angles, in some cells which combine water- 
storage with strengthening (figure 50, /); and it takes various 
other forms too many to mention. 

Furthermore, the composition of the wall is alterable both 
physically and chemically. Cellulose is a very elastic substance, 
and where greater stiffness than it can afford must be had, the 
wall becomes penetrated by the far stiff er substance lignin; and 
lignified walls are wood. Both cellulose and lignin, however, 
allow ready passage of water, and where that would be a danger, 
as at surfaces of plants which grow in dry air, the wall is made 
waterproof by the formation all through its texture of a water- 
repelling substance, called cutin or suberin; and such is the case 
with the epidermis and cork which form the skin of plants. In 
other cases the wall softens to mucilage on the access of water, 
as in Flax seeds, though the reason thereof is not perfectly clear; 
and there are yet other such modifications of more special char- 
acter and meaning. 

It is thus plain that cell-walls are well-nigh indefinitely plastic 
in shape, thickening, and composition, while, moreover, any and 
all of these features can be combined in various ways and de- 
grees in accordance with the particular needs or functions con- 
cerned. Furthermore, the cells are rarely isolated; but commonly 
cooperate in large masses of similar function called tissues. 
Masses of tissues cooperating in function, and mutually adjusted 
to perform their work to the common advantage, form organs, 
and organs make up the plant. 

There remains one other matter of importance about the wall. 
Although, at first sight, it seems to shut off completely the proto- 
plasm of each cell from that of its neighbors, minute observation 
exhibits the presence of definite thin places perforated by very 
fine pores which permit the passage of tiny threads of living proto- 
plasm from one cell to another (figure 51). This continuity of 
nrnt.nnlasm from PC*\\ t.n ppll Vm.s hppn found in fvrorv nart, of the 



158 The Living Plant 

plant, where it has been sought; and it seems clear that every 
living cell is thus in communication with its neighbors, and 
therefore with every other living cell of the plant. Thus the 
protoplasm though partially, is not wholly, separated into cellu- 
, , f :. lar masses, and is, after all, for any 

individual plant a single great con- 
tinuous sheet. There is every reason 
to believe that impulses of different 
kinds can be transmitted from cell 
to cell through these threads, which, 
therefore, take the place in part of 
the nerve system of animals. This 
helps us to understand how it is that 
the plant can act as a physiological 
^'?!v : unit: how the different parts of a 
plant can be kept in harmonious 
FRJ. 51. An ordinary cell specially cooperation: and how stimuli applied 

treated to show the thin threads 

of protoplasm extending through at one part oi a plant, can produce 

the wall to connect its protoplasm . * > , , i i i T 

with that of its neighbors. (Copied their effects at a considerable dis- 

from Strasburger's Lehrbuch). tiRCP 

Thus much for the wall of the cell, to which, it may seem to the 
reader, I have devoted a disproportionate space and attention. 
Yet while vastly less important than the protoplasm, the solidity, 
prominence, and relative permanence of the walls makes them far 
more accessible to study, to such a degree indeed that our con- 
ceptions of cellular structure center much more largely around 
the walls than the protoplasm.* This, however, is less unfortu- 

* The inconspicuousness of the living protoplasm of plants in comparison with 
the prominence of the walls it builds finds striking exemplification in the history of 
their discovery; for the mass of the walls was well described, and their cavities were 
named cells, by Robert Hooke as early as 1667, while they were elaborately described 
and beautifully pictured only a few years later, 1672-1682, in the fine books of Grew 
and Malpighi. But the protoplasm was not recognized at all as a constituent of 
cells until over a century and a half later, and was only first adequately described 
and named by von Mohl, in 1844. 

Here is Ilooke's sentence, of 16(57, in which cells were first named. He is describ- 



The Substance Which Is Alive in Plants 159 

nate than it might seem, because the constitution of the walls 
is so closely interlocked with the functions of the cells that from 
the one we can infer much as to the other. 

Passing now to the cytoplasm we can briefly dismiss it, for, 
being the typical protoplasm, it has already been fully described 
in the earlier part of this chapter. It is the working body of the 
cell, concerned with its nutrition, construction, etc., and the 
streaming movements are probably concerned with the trans- 
portation of substances through the cell, a view sustained by the 
fact that the streaming is most active in general in the cells which 
are largest. The cytoplasm does not differ particularly in appear- 
ance in different cells, excepting that it is more fluid in some and 
more solid in others. One point of present interest about it, how- 
ever, is this, that just at this time of writing, certain newly found 
tiny bodies within it, called mitochondria, or chondriosomes 
are attracting much attention, and may prove to be very im- 
portant. 

We come next to the nucleus of the cell. It consists of living 

ing the appearance presented by a thin section of cork placed under his microscope. 
"1 could exceeding plainly perceive it to be all perforated and porous, much like a 
Honey-comb, but- that the pores of it were not regular; yet it was not unlike a Honey- 
comb in these particulars. 

First, in that it had a very little solid substance, in comparison of the empty cavity 
that was contain'd between, . . . for the Inlertstitui, or walls (as I may so call them) 
or partitions of those pores were neer as thin in proportion to their pores, as those 
thin films of Wax in a Honey-comb (which enclose and constitute the sexangular cells) 
are to theirs. 

Next, in that these pores, or cells, were not very deep, but consisted of a great many 
little Boxes, separated out of one continued long pore, by certain Diaphragms, . . . 

I no sooner discern'd these (which were indeed the first microscopical pores I ever 
saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, 
that had made any mention of them before this)." . . . (Robert Hooke, Micro- 
r;w/>/ua, 1665, 113.) 

Here is von MohFs sentence, of 1844, in which protoplasm was first named: "So 
mag es wohl gerechtfertigt sein, wenn ich zur Bezeichnung dieser Substanz eine 
auf diese physiologische Function sich beziehende Benennung in dem Worte Proto- 
plasma vorsehlage." (ttotanixche Zritung, 1844, page 273); or, in translation, " Ac- 
cordingly it may be justifiable if for designating this substance I propose an appellation 
having reference to this physiological function, namely, the word Protoplasm." 



160 The Living Plant 

protoplasm, denser than the cytoplasm, and different, somewhat, 
chemically. It varies comparatively little in appearance in differ- 
ent cells, and ordinarily exhibits no particular structure; but when 
the cells are dividing or reproducing, then a definite number of 
rod-shaped structures become differentiated and perform re- 
markable manoeuvres which we shall later consider in the suit- 
able place along with reproduction (figure 101) . These rods, called 
chromosomes, are the seat of the controlling power of heredity, 
and thus guide the constructive work of the cytoplasm in growth. 
The nucleus, therefore, bears to the cytoplasm a relation sug- 
gestive of that of the brain to the body. Indeed, the resemblance 
may extend pretty far, since there are those who maintain that 
heredity in the chromosomes is substantially the same thing as 
memory in the brain. But I hope the reader will not therefore 
call the nucleus the brain of the cell, for it isn't. As to the nu- 
cleolus, that is irregular in its appearance, and probably repre- 
sents a reserve of chemical substance for use in the growth of the 
chromosomes. 

The plastids, likewise, are living protoplasm, and are present in 
all cells of the typical plants, though sometimes they are incon- 
spicuous. Thus, it is the plastids which hold the green color 
in leaf-cells, where they are already well known to the reader as 
chlorophyll grains, called also chloroplastids. In other cells, 
of some fruits, such as the familiar Jerusalem Cherry, they con- 
tain yellow or orange colors (chromoplastids), thus aiding to 
make the fruits conspicuous. And in other cells yet, especially 
in the storage parts of plants, they remain colorless and are called 
leucoplastids, but perform the remarkable and indispensable 
function of converting sugar to starch. It is, indeed, a fact of 
the greatest interest about these leucoplastids, that they and the 
homologous chloroplastids comprise the only places in nature, 
either within the plant or outside of it, where starch is known to be 
made. Starch is one of the substances which the chemist has not 
yet been able to make in his laboratory. 



The Substance Which Is Alive in Plants 161 

The sap-cavities are as simple in structure as they look. In 
very young cells they are absent, as a later picture illustrates 
(figure 129) ; but in those that are older little rifts appear in the 
cytoplasm and gradually grow larger until finally, in the fully 
mature cell, they become merged into a single cavity of very large 
size, as the figure of the conventionalized cell clearly shows 
(figure 48). This cavity is filled with water in which sugar and 
other useful substances are dissolved. It thus represents a store- 
house of useful materials, but serves secondary functions, like- 
wise, in pressing the cytoplasm against the wall, and in aiding 
growth, by methods which will later be described in the suitable 
chapters. 

It is of interest to note that not only does new protoplasm in 
general originate only from preexisting protoplasm, but new 
cells originate only from cells, nuclei from nuclei, and plastids 
from plastids; while the same thing has been claimed even 
for cell-wall and sap-cavities, or rather for the part of the 
cytoplasm which forms them, though here the evidence is not 
conclusive. 

The question is now appropriate, why does protoplasm sep- 
arate into cells at all, and what makes them of such minute size 
as they are? It is sometimes assumed that the plant-structure 
becomes cut up into cells in order to provide structural units of 
convenient size and form, after the manner of the bricks of the 
builder; but the analogy is wholly misleading, since the skeleton 
of plants is not built at all from originally separate units, as brick 
buildings are, but rather from a continuous mass of cell-wall 
substance comparable with the cement construction now coming 
into use. Another explanation maintains that each nucleus 
can control only a limited quantity of cytoplasm; and thus are 
established certain administrative units between which, natu- 
rally enough, the walls are built, the resultant being cells. As 
to the reasons why their sizes are so small as to require a mi- 
croscope to show them at all, we have again a few guesses, but no 



1 62 The Living Plant 

exact knowledge. Possibly the chromosomes need a certain 
size in order to perform their functions; this would establish the 
size of the nucleus, and hence (on the explanation above noted) 
of the cell. Another explanation rests on a mathematical basis. 
We may assume that the typical cell is a sphere filled solidly 
with protoplasm. When a sphere enlarges in size, its bulk in- 
creases much faster than its surface, the bulk increasing as the 
cube of its diameter and the surface as the square thereof. Ob- 
viously it is through this surface that the spherical cell must 
absorb the oxygen for the respiration of the entire bulk of its 
protoplasm; it is therefore quite evident that there must be a 
certain size of the cell in which the surface is just sufficient to 
aerate the bulk of protoplasm within, and that size would de- 
termine the average cell size. If the cell were to grow larger its 
surface would not suffice to aerate the bulk, while if smaller the 
surface would be needlessly great. In a general way this con- 
clusion is sustained by the fact that where conditions for respira- 
tion are harder the cells are smaller, and vice versa. Moreover, 
the very largest cells occur in places well situated for aeration, 
and besides, possess accessory arrangements, viz., the flattening 
of the protoplasm in a thin layer against the wall (figure 45), and 
protoplasmic streaming, which aid to that end. These features 
prevail in the hair cells already observed by the reader, and in 
consequence those cells become large enough to be visible to the 
eye without the aid of a lens. In general, therefore, it does seem 
true that the relation of bulk to surface in a solid as affecting res- 
piration is one of the principal factors, if not indeed the principal 
one, in making the size of cells what it is. 

In comparing the functions of the cells of plants with those in 
animals, it soon becomes obvious that plant cells exhibit a far 
lower degree of division of labor; and this involves a remarkable 
consequence. It seems to be a fact that when protoplasm con- 
tinues to perform a single function for long periods of time, as it 
does in the highly-specialized organs of the animal body, it grows 



The Substance Which Is Alive in Plants 163 

stronger and stronger and works better and better up to a certain 
culminating level, beyond which it tends to decline, and finally 
to cease work altogether. It is probable that the decline and 
cessation of work is dependent upon purely physical causes, some- 
what as a bar of metal when too often bent, becomes weakened 
and broken at last; but in this peculiarity of protoplasm we find 
an explanation for the cycle of youth, maturity, old age and death. 
When, however, the protoplasm can periodically alter its location, 
habits, or functions, can re-melt itself, so to speak, it renews 
its youth thereby, and can continue its vigor without limit, thus 
becoming potentially immortal. In this fact is found the ex- 
planation of the benefit wrought by a change of scene or occupa- 
tion, or a vacation, upon ourselves, though the effect is here 
limited; and if a way could be found to affect our protoplasm 
more profoundly, to make it mix itself up periodically, even 
within the limits of the same cell, then, it seems likely, man 
would have discovered the long-sought elixir of life and the 
secret of perpetual youth. This in fact is the case in full degree 
in simple plants like Bacteria. Each of these is made of one cell, 
and when it reaches full size divides into two, each of which 
grows up and divides again, and so on without limit, in perennial 
change, vigor, and youth. A similar rejuvenation takes place in 
sexual reproduction, when the protoplasm of two individuals 
mingles together in fertilization. Now, the higher plants possess 
no organs at all in which the protoplasm continues to work within 
the same cells throughout the life of the individual, but, as our 
chapter on growth will abundantly illustrate, the protoplasm is 
continually moving outward and onward into newly forming 
buds, leaves, roots, and stems; and this removal permits it to re- 
new its youth perennially. Therefore plants should never grow 
old from internal causes, in the way that animals do, and in 
fact they do not, the exception presented by annuals being only 
apparent and not real. Even the greatest trees continue to form 
new leaves and roots with unabated vigor until they are brought 



164 The Living Plant 

to their death by external causes, chiefly connected with the 
large sitfe they attain. 

The manifestations of life, wherever we know them, are as- 
sociated closely with constant changes of matter and energy, 
especially with respiration. But there is a case in which all of 
these processes seem suspended, for our most delicate methods of 
research fail to demonstrate them, and that is in resting struc- 
tures such as seeds, Resurrection plants, and some low animals. 
Not only can dry seeds retain their vitality for a great many years, 
but in that condition they can withstand without injury a tem- 
perature above boiling point, or even two hundred degrees below 
freezing point. The question is important whether the usual 
changes are proceeding in these seeds, but too slowly to be meas- 
ured, or whether all processes stop and the vitality is really sus- 
pended. The truth is not as yet known, but it is to be noted that 
there is no logical difficulty in supposing that all of the processes 
may slow down to a stop without any derangement of machinery, 
precisely as an engine is stopped for the night simply by with- 
holding the steam, leaving it all ready to start once more in the 
morning. 

When this chapter was finished down to this point, it was 
handed like all of the others to a critic for judgment. And this 
is in substance the comment with which it came back. "The 
chapter is clear enough in its statements, and appears to cover 
the subject, but somehow it leaves you with a very unsatisfied 
feeling." This opinion I take for a very high compliment, since 
it shows that my chapter reflects precisely the scientific situation 
of the subject. 



CHAPTER VII 

THE WAYS IN WHICH PLANTS DRAW INTO THEMSELVES 
THE VARIOUS MATERIALS THEY NEED 

Absorption; Roots 




N the preceding chapters we have traced pretty fully 
the principal processes occurring within the bodies of 
plants. But as yet we have taken no thought of the 
ways in which plants absorb the various materials they 
need from outside; and this is the inquiry which now lies before 
us. I give the reader fair warning that the subject will lead us 
perforce into distant, unfamiliar, and recondite matters; but their 
study will have the advantage of illuminating a good many things 
besides absorption by plants. 

Of all the substances absorbed by plants, the foremost is water, 
which not only makes up a great part of plant substance, but 
is also indispensable for various physical and chemical uses. 
This water is absorbed, as everybody knows, through the roots; 
but the fact is less familiar that the absorption takes place ex- 
clusively through the young white terminal parts. Upon these, 
accordingly, we now center our attention. They can be studied 
most easily, and without obscuration by adherent soil particles, 
in young roots obtained by the germination of seeds in flower-pot 
saucers kept shaded and wet. From such specimens it appears 
that young roots as a whole are remarkably alike, and possess 
several features in common, viz., a slender white shaft with a 
yellowish tip, and a diaphanous garment of delicate radiating 
hairs, features which are shown very well, except for the color, 
in the seedling of Mustard pictured herewith (figure 52). If 

165 



1 66 The Living Plant 

one centers observation more exactly on these hairs, he will see 
that nearest the tip they are plainly just forming, while farther 
back they are progressively longer, until a maximum is reached, 
behind which they are obviously withering and dying. Evi- 
dently the single hairs have each their little day 
and pass, while the zone as a whole moves forward 
in perpetual youth, pari passu with the advancing 
tip of the root. These hairs are of first importance 
to our immediate subject, for they are the active 
water-absorbing parts of the roots. 

Thus much can be seen with the eye and a 
lens, but hardly anything more. If, however, one 
cuts a thin section through the apex and along the 
central axis of a root, and magnifies this section 
. with the microscope, he will have before him an 

Fio. 52. A seed- l ' 

ling of mustard arrangement like that of our picture (figure 53), 

grown in dark . , . . , . ... , , . , , , 

saturated air; with which it will be desirable to compare the 
naturalize. generalized section of figure 139 C. At the tip is 
the root-cap, a cluster of cells which, continually renewed from 
behind, acts as a protection to the delicate tip in its passage 
through the rough and abrading soil; just behind lies a prominent 
focal center, the growing point, whose closely-packed cells are so 
densely filled with protoplasm that the characteristic yellowish 
color of that substance shows through to the outside; while radi- 
ating back from the growing point run long lines of cells which 
gradually merge into the differentiated tissues of the older root. 
These latter tissues, so far as they concern our immediate subject 
of absorption, are shown generalized in figure 54, D. Of the lines 
of cells, a few in the center constitute the pith, outside of which lie 
the long lines of water-tubes, or ducts, readily identified by their 
distinctive spiral markings. These ducts contain water, but, 
contrary to what one would expect, are otherwise empty tubes, 
possessing no living protoplasm after once they are formed; and 
they run in continuous strands from the tips of the roots all 




Fi(}. ,53. Typical parts of a section drawn lengthwise, cell for cell, through a young 
root of Corn. The entire section is not presented because its length would be 
much too great for the page. The tip portion is magnified more than the others. 



Longitudinal section through stem at y. 




D, Longitudinal section through a portion of root at *, 



Fio. 54. Generalized drawings illustrating the absorbing and conducting systems of the 

plant. 

168 



How Plants Draw in Various Materials 



169 



through the stems to the leaves, as shown very clearly in the con- 
ventionalized plant of figure 54, A. Outside of the ducts lie some 
rows of rounded-elongated cortical 
cells, each of which retains its lining 
of protoplasm and is shown by tests 
to contain a solution of sugar. Finally, 
outside of all lies the single thin line 
of epidermal cells, which display a 
very striking feature, viz., a great 
many are prolonged into slender cy- 
lindrical closed tubes, which are ob- 
viously identical with the root hairs al- 
ready observed in the young living 
roots; and each hair is lined by living 
protoplasm and, as shown by suitable 
tests, contains a solution of sugar. 
Now the structures important from 
the view of water-absorption are the 
ducts, the cortical cells, and the root 
hairs; and these parts constitute the 
water-absorbing machine. And this 
machine, if reduced to a single cell of 
each kind, would be constructed some- 
what as suggested by figure 55. 

We turn now to the forces con- 
cerned in absorption. Most people, 

if questioned, WOUld doubtless express Fl(J - 55.A diagram of the con- 
struction of the water-absorbing 




the belief that roots suck up water in 
much the same manner that a wick 
sucks up oil, that is, by the power, 
called in physics capillarity) the same 
which takes liquids up fine tubes. In- 
deed this was once the belief of botanists themselves, as witnessed 
by their former use of the term "spongiole," that is "little 



machine, as it would appear if 
reduced to a single root hair, 
cortical cell, arid duct. Pro- 
toplasm is shaded; circles are 
water; crosses are sugar; the ar- 
rows show direction of move- 
ment. Magnification as in Fig. 6. 



170 The Living Plant 

sponge/' for the tip of the root, which was supposed to soak up 
water and pass it on to the ducts. Later it was found that the 
water enters chiefly through the hairs. But in these the condi- 
tions for capillarity are absent; for capillarity requires openings, 
and the hair walls contain none that even the most powerful mi- 
croscopes can detect. The problem is, therefore, to explain an 
absorption of water through membranes that are imperforate, or 
solid, and through protoplasm-lined and sugar-holding cells into 
protoplasm-less and sugar-less ducts. But the very mention of 
absorption into sugar solutions through imperforate membranes 
immediately suggests a direction for our further inquiry, since it 
recalls a mode of absorption very well known in physics, and as- 
sociated with those very conditions, viz., Osmosis. 

So important is this subject of osmosis to an understanding not 
only of absorption of water by plants, but of many other notable 
phenomena as well, that the reader ought really to make its more 
intimate personal acquaintance. This he can do by aid of the 
following experiment, which is one familiar to all workers with 
plant physiology. Over the end of a large glass tube is tied 
firmly, by means of waxed thread, a piece of soaked parchment, 
(preferably a cylindrical parchment cup made for the purpose) 
which is a physical equivalent of the wall of the root hair; into the 
tube is poured a solution of sugar, for which molasses, a solution 
ready made and conveniently colored, is excellent; then the tube 
is supported with the parchment in pure water, the whole arrange- 
ment being much as displayed in the accompanying picture 
(figure 56). A surprising result always follows, for, without the 
operation of any visible machinery or forces, and in a manner 
which to me, despite long familiarity therewith, looks always 
anomalous and even somewhat uncanny, the liquid rises steadily 
though slowly in the tube, lifting its own considerable weight, 
until within two or three days it has reached a height of two or 
three feet or more. Indeed the process can be demonstrated 
even more strikingly than this, for, if the parchment cup be 



How Plants Draw in Various Materials 



171 



made very large and the glass tube very small, as is readily ar- 
ranged for purposes of demonstration, the liquid will mount stead- 
ily up before the very eyes to a height of several feet. Obviously 
there is only one possible explanation of the rise of the liquid 
against gravitation, viz., water must pass through the parch- 




FIG. 56. An osmoscopc, using a parchment membrane; further particulars in text. 

ment, and that not simply in a manner that is passive, but with 
a force sufficient to overcome a considerable resistance. The same 
result invariably follows, with a difference, however, in the rate 
of the ascent, no matter what solution is put inside of the tube, 
and follows, moreover, in case there is also a solution outside, if 
only the inner solution is the stronger. This is a typical example 
of osmosis under its simplest conditions, but it is representative of 



172 



The Living Plant 



all conditions, inside of plants and animals, as well as outside of 
them. It thus constitutes one of the great natural verities which 
may be stated as follows; when water and a solution, or two solu- 
tions of different strengths, are separated by a suitable membrane, 

there is always a forcible osmotic 
movement of liquid through the mem- 
brane from the weaker to the stronger 
solution. This is one of those ele- 
mental cosmical facts which the 
reader should fix in his mind as one 
of the pillars of his natural knowl- 
edge. 

If, at this point, it seems to the 
reader that however interesting such 
experiments with parchment and 
tubes may be, they can have little 
to do with the processes inside of a 
living plant, let him take a leafy 
potted Begonia, Fuchsia, or Mar- 
guerite, cut everything away close 
down to the roots, and connect the 
stump with a plain glass tube like 
that which was used in the foregoing 
experiment. Then, I believe, he will 
change his opinion, for water always 
rises in the tube, though slowly, to 

arrangement in which ^ ^^ ^ ^ Qr ^^ f ^ (figure 




replaced by living roots. 57) . There are Qf courge p l en ty o f 

differences in detail, but sugar-holding cup and live roots agree in 
the central and crucial feature that they absorb water into a sugar 
solution through imperforate membranes and force it up tubes 
against gravitation. There is no question that the primary 
forces are the same in both cases, and that the absorption of 
water by roots is osmotic. 



How Plants Draw in Various Materials 173 

We return for a moment to our osmoscope, for such is the name 
of our osmosis-exhibiting instrument. As the liquid ascends in 
the tube, a brown color appears in the water outside, showing 
that some of the molasses comes out, though of course in much 
smaller amount than the water which enters, else the liquid could 
not rise in the tube. This suggests at once the inquiry, does the 
sugar in the sap of the root-hair cells also come out into the soil? 
It does not, as ample evidence attests. And if we seek in parch- 
ment cup and root hair for a structural difference to explain this 
difference in osmotic action, we can easily find it; for the hairs 
possess a complete lining film of living protoplasm to which 
there is no equivalent in the parchment cup. It is easily shown by 
experiment that this protoplasm really does stop the passage of 
sugar while permitting that of water; and this fact explains not 
only why no sugar passes out of the hairs into the soil, but also 
the equally striking phenomenon, that none passes out of the 
cortical cells into the ducts, for in general it is only pure water 
which ascends through the ducts to the leaves. Protoplasm, 
however, is not the only membrane of this type (which, because 
permeable to water but not to dissolved substance is called semi- 
permeable, in distinction from the ordinary kind which are per- 
meable to both), for they can be constructed artificially from chem- 
icals, and even laid down in a uniform film all over the interior 
face of the parchment cup. In this case our osmoscope becomes a 
very close physical duplicate of the living root hair, and likewise 
permits the steady absorption of water without the escape of any 
of the sugar whatsoever. My students have often constructed 
such arrangements, with results that were wholly satisfactory. 

If, now, the reader will compare point by point, an osmoscope 
containing a semipermeable membrane, and the absorbing mech- 
anism of the living root (which is diagrammatically represented 
in figure 55), he will agree that they match very closely in physical 
construction and operation except for one very notable difference, 
namely, while the liquid which rises in the osmoscope is a mix- 



174 The Living Plant 

turc of molasses and water, that which rises in the ducts is prac- 
tically pure water. This difference, obviously, is correlated with 
a difference of structure, viz., in the plant the water has to pass 
through intermediate cells, which are wanting in the osmo- 
scope. We have already learned why it is that the sugar does 
not pass with the water into the ducts (the protoplasm stops 
it), and our problem resolves itself into this, how is it that 
the cortical cells send water into the ducts when all of the con- 
ditions seem rather to invite the absorption of water from them, 
exactly as the hairs absorb it. from the soil? This question, 
I am sorry to say, I cannot yet answer, for it remains one of the 
unsolved problems of plant physiology, though one of the most 
inviting of them all. It is true, some physiological books at- 
tempt to explain it, but in all cases, so far as I have observed, 
either their physics is bad, or else their explanations are worded 
in a manner more lethal than logical. I suspect the explana- 
tion will ultimately be found in some ordinary physical or chem- 
ical processes working under control of some still unknown prop- 
erty of the protoplasm. 

In the three or four paragraphs which follow I purpose to 
explain how it is that substances in solution can pass through 
imperforate membranes, and what are the forces which drive 
them. The subject involves a consideration of molecules, and 
things of that sort, and will require hard work from the con- 
structive imagination. So the reader is given fair warning and 
may skip if he pleases, though I beg to remind him that this 
book makes appeal to his reason, and is an attempt to help him 
to share the spartan pleasures of understanding. 

The most striking feature of osmotic absorption consists in 
the remarkable rise of a large body of liquid against gravitation 
without the operation of any visible forces whatsoever. Yet 
forces there must be, if not visible, then invisible; and accord- 
ingly we turn for the sources of the power deep within the con- 
stitution of the bodies themselves. As everybody knows, mem- 



How Plants Draw in Various Materials 175 

branes, water, and dissolved substances arc all of them composed, 
according to the teaching of physics, of ultimate excessively 
small units, called molecules. In the solid or liquid state, the 
molecules are held together by a force of mutual attraction, 
called cohesion, analogous to the force which holds an armature 
to a magnet. But when heat in sufficient amount is supplied, 
there comes a point at which the cohesion of the molecules is 
suddenly overcome and replaced by an opposite tendency to 
spread or diffuse just as far apart as they can; and this is what 
constitutes a gas. The power that actuates the diffusion is heat, 
which, catching the tiny molecules in the swirl of the violently- 
vibratory ethereal waves of which it consists, imparts to them 
its own vigorous motion, whereby they are set swiftly darting 
and dancing hither and yon, bounding and rebounding energet- 
ically against one another, with a result that they work steadily 
outward, very much as a cargo of corks would be spread from a 
foundered vessel on the waves of a tempestuous sea. Familiar 
examples of this diffusion of gases are many, for instance, the 
spread and ultimate disappearance of odors, and the penetration 
of cigar smoke though the house; but all gases diffuse in this 
manner. And here comes a curious and consequential fact about 
diffusion, namely, that it occurs not only in gases, but also in 
anything, whether solid, liquid or gaseous, when dissolved in a 
liquid. Examples thereof are abundant, the gradual spread of 
a bit of solid dye when dropped into water: the spread of sugar 
through coffee or tea without stirring if only time be allowed: 
the spread of fertilizers evenly through soil though added in large 
lumps on the surface. By diffusion, also, the molasses reaches 
the water outside of the tube of our osmoscope. Such diffusion 
occurs, as it seems, because an adhesive attraction existing be- 
tween the molecules of the substance and those of the dissolving 
liquid separates the molecules of the substance from one another, 
and thus brings them into a condition such that heat can exert 
upon them the same action as it does upon the separated mole- 



1 7 6 



The Living Plant 




cules of a gas (figure 58). And if the reader objects at this 
point that diffusion in a solution takes place at a temperature 
too low to permit this explanation, I remind him that days far 
too cold for our comfort are yet hot from the physical point of 

view, for there is heat in the air 
at all temperatures above the ab- 
solute zero, which lies no less than 
four hundred and fifty-nine de- 
grees below zero of our ordinary 
thermometer. And the phenomena 
of diffusion are precisely the same 
FIG. 5s. A diagram designed to aius- inside of plants and animals as 

tratc the diffusion of a substance in 

solution. The circles are water, and OUtSlde of them. We are nOW 

the crosses are the dissolving and dif- i , T/V 

fusing S ubstance,-c. K ., sugar. The prepared to summarize diffusion 

molecules of water arc supposed to &s ano ther Verity of nature, thus, 

have a stronger attraction for the J ' ' 

molecules of sugar than these have for when Substances (1TC anywhere 

one another. Magnified as in Fig. 6. 

brought into a state, whether by 

conversion to a gas or by solution in a liquid, such that their mole- 
cules are separated from one another, then those molecules, set into 
energetic action, and thereby given a mutually-repulsive motion, by 
heat derived from the surroundings, spread, or diffuse, forcibly out- 
ward from places of greater to those of lesser concentration. 

Thus much for diffusion; we turn next to the other condition 
involved in osmosis, the nature of the membrane. What can 
be the constitution of a body which, possessing no discoverable 
openings, will permit water and other substances to pass through 
with a freedom well-nigh as uncanny as if a fourth dimension 
were concerned? The membrane, of course, is composed of mole- 
cules, but there is also good reason to believe that, in walls at least, 
the membrane is composed of larger units, called micelles, which 
are aggregates of molecules (or perhaps simply huge compound 
molecules) that may be represented diagrammatically as cubical 
(figure 59). Now these micellae, although structurally separate, 
are held closely together by virtue of a certain cohesive affinity 



How Plants Draw in Various Materials 



177 



for one another, somewhat as a magnet and its armature are 
held together by magnetism; and this explains why the mem- 
brane, although composed wholly of separate units, holds to- 




FIG. 59. A diagram illustrating the construction of membranes. The circles are water; 
the smaller squares are molecules, arid the larger arc micellae, of wall substance, a, rep- 
resents a dry membrane (which always contains some water) and b, a saturated mem- 
brane, supposed to be seen in section, reduced to only a few micellae. 

gether as a solid. At the same time the micellae possess a still 
stronger affinity for something quite different, namely water, 
which accordingly they can draw in as thin films among and 



178 The Living Plant 

around themselves, thus forcing themselves apart against the 
resistance of their own cohesion. This explains how it is that 
membranes, and all bodies of similar constitution, like wood, can 
forcibly absorb water throughout all of their structure, and 
swell up in the process, the requisite energy being supplied by 
the adhesive attraction between water and wood. This inter- 
micellar absorption of water is called imbibition, and is represented 
in the accompanying diagram (figure 59). But why, by the way, 
are the micellae not driven entirely apart by the water, thus 
making the membrane completely soluble therein? The reason 
is believed to be this, that while the adhesive attraction of 
micellae for water, and the cohesive attraction of the micellae for 
one another, are, like the attraction of a magnet for its armature, 
strongest when the parts are the closest and weaker with increas- 
ing distance apart, the adhesion is supposed to weaken with 
distance more rapidly than the cohesion; hence, although the 
adhesion between micellae and water is at first stronger than the 
cohesion of the micellae (thus drawing in some films of the water) 
there soon comes a point at which the rapidly-lessening adhesion 
between water and micellae exactly balances the slowly-lessening 
cohesion between the micellae, and this point of equilibrium is 
that where the membrane is saturated with water and swollen 
its greatest, as supposed to be represented in figure 59, 6. In this 
condition the intermicellar spaces will possess a certain definite 
size, differing, of course, with the nature of the membrane; and 
in these different sizes we find the simplest explanation of the 
different behavior of the types of membranes, for the semi- 
permeable would be one with intermicellar spaces too small to 
allow the sugar or other large molecules to pass, while giving free 
transit to the much smaller water molecules, while the permeable 
has large enough spaces to permit both kinds to pass. The case 
in reality, however, is not quite so simple as this, for plenty of 
facts show that adhesion or solution relations between dissolved 
substance and the membrane play also a part. Moreover, the 



How Plants Draw in Various Materials 



179 



condition of balance in a saturated membrane explains how it 
is that water can pass so readily through it; for the last films 
absorbed, those farthest from the micellae, are held so very 
lightly that only a slight force is required to draw them from the 
membrane. What the nature of the 
force may be which withdraws the 
water from the inner face of the mem- 
brane in osmosis we shall consider in 
a moment. 

Diffusion, imbibition, osmosis it- 
self are typical examples of molec- 
ular forces, those operating between 
individual molecules, in contrast with 
the more familiar molar forces which 
act upon masses. There is also one 
other molecular force of some im- 
portance in the plant, viz., capil- 
larity, which we must now briefly 
notice. Capillarity is the well- 
known force by which water is raised 
in small tubes, or any small pas- 
sages no matter how irregular, and 
the higher the finer the tubes, as our 
diagram illustrates (figure 60). It 
is the power by which a towel dries 
water from the skin, a blotter takes up ink, a wick raises 
oil, or any porous substance soaks up liquids. It is only with 
difficulty, and under suasion from my critic, that I forbear to 
explain this interesting process in detail to the reader; and I 
must regretfully confine my exposition to the following brief 
synopsis. The capillary rise of water is due to forces residing 
within the water itself. Because the attractions mutually exerted 
between the molecules inside of the liquid are not balanced at 
the surface by equivalent attractions towards the outside (fig- 




d. GO. A diagram to illustrate the 
rise and depression of liquids in 
capillary tubes, drawn to approxi- 
mately true scale. The liquid 011 
the left is mercury, and on the right 
is water. 



i8o 



The Living Plant 



ure 61, 6), the surface layers of molecules are drawn strongly 
inward so that collectively they press on the liquid as if they were 
tightly stretched rubber, a phenomenon known as surface 
tension. Now surfaces that are flat press inward with a definite 
force, but those which are concave, being partially buried, as it 
were (figure 01, c), within the body of the liquid, and therefore 
having the inward attractions of the molecules a little com- 






et 6 c 

YLG. 01. A diagram to illustrate the? operation of forces concerned in capillarity, repre- 
senting sections through convex, flat, and concave water surfaces. The small circles, 
open and solid, are water molecules, and the larger circles are the areas within which 
given molecules, represented black, are cohesively attracted by others. Where; these 
areas lie wholly within the liquid, as shown in the lower part of b, the attractions 
balance? one another, and no effect is produced; but where the areas fall partly outside 
of the liquid, the inward attractions are not resisted by equivalent outward ones, 
though the exact degree thereof depends on the form of the surface. 

pensated by partial attractions outward, press inwards with less 
force, while those which are convex, projecting as it were outside 
of the liquid, have their molecules drawn in with an even stronger 
attraction than have those of a flat surface (figure 61, a). There- 
fore it follows that the very mobile water will always be pressed 
away from flat or convex surfaces towards those which are con- 
cave. Now it happens, furthermore, that water adheres both to 
glass and to wood, and hence in a tube of either of these substances 



How Plants Draw in Various Materials 181 

climbs up a bit on the wall, as our figure 61, c well illustrates, mak- 
ing the surface concave to a degree that is greater the smaller the 
tube. Hence the greater surface tension of the flat surface outside 
pushes the mobile water up against the lesser pressure of the con- 
cave surface inside, forcing it to rise against gravitation until equi- 
librium is established, which will occur at a higher point the smaller 
the tube. And the reverse process occurs with liquids which 
will not adhere to glass or wood, e. g., mercury, or with walls 
of such composition that water will not adhere thereto, as in 
some air passages of plants; for in this case the surface in the 
tube is convex, and presses the water down against the flat sur- 
faces outside, so that the liquid stands below the outside level in 
the tube (figure 60, on the left), or, if the tube is not deeply im- 
mersed, will not enter at all. 

Such is capillarity, deriving its energy from internal molecular 
tensions given release by peculiarities of external conditions, and, 
like all molecular forces, strictly limited in amount and without 
possibility of continuous action. Capillarity plays in the plant 
some minor part in the ascent of sap, in prevention of the entrance 
of water into some air passages, and in other processes later to be 
noted. Moreover, some physicists see in imbibition nothing but 
a refined capillarity, although as I think, the phenomena of im- 
bibition of water vapor, presently to be noted under hygrosco- 
picity, is hardly consonant with this explanation. Still another 
possible connection of a refined capillarity with osmotic absorp- 
tion will be noticed in a moment. 

We have now reached the place where the reader who may 
have used my permission to skip for a little must resume his 
grasp on this narrative if he is to understand the essentials of 
osmotic phenomena. 

In watching the ascent of a liquid in an osmoscope, like that 
of figure 56, one sooner or later comes to wonder what would 
happen in case an insuperable barrier, e. g., a tight stopper, were 
interposed against the further rise of the liquid. The matter is 



182 



The Living Plant 




IM<;. 02. Pfeffer's coll, as 
pictured by himself in his 
own book (but reduced 
to ?<( his si/e). 

A semipermeable mem- 
brane is formed all over 
the inner face of the 
porcelain cup, which is 
shown, in half, at the 
lower right of the figure. 
The cup, and all the re- 
mainder of the appara- 
tus, is then filled with 
the sugar s o I u t i o n, 
which, absorbing water 
when the cup is im- 
mersed, presses the mer- 
cury up against the air 
in the gauge to a height 
which balances, arid 
measures, the pressure. 
The remaining mechani- 
cal features are con- 
nected with filling arid 
sealing the cup. 



easy of experiment and the answer plain; 
the cup becomes stretched or even pushed 
from the tube, or sometimes (and always if 
provided with a semipermeable membrane) 
it bursts. This shows that osmotic ab- 
sorption, if confined, develops osmotic 
pressure. Of course the pressures have 
been measured exactly, chiefly by aid of an 
instrument invented by the great botanist 
Pfeffer, and shown by the accompanying 
picture (figure 62). When its porous cup, 
lined with a semipermeable membrane, is 
filled with a solution of sugar like that in 
root hairs, and then is immersed in pure 
water, the gauge actually exhibits a pres- 
sure equal to that of. three or four atmos- 
pheres, or fifty to sixty pounds to the square 
inch. Nor is this all, for when very strong 
solutions are used, which require, of course, 
an instrument of enormously greater 
strength, pressures of surprisingly high 
magnitude have been registered, even up to 
twenty-four atmospheres, or 360 pounds to 
the square inch, a much higher pressure 
indeed than ever is used in the steam boilers 
of even the swiftest express locomotives; 
while recently even higher ones have been 
measured. Nor are such pressures of merely 
academic interest to the botanist, because 
others higher yet, above one hundred at- 
mospheres, have been found to exist under 
special conditions in plant cells. 

Here follows another paragraph which the 
reader may skip if such be his inclination, 



How Plants Draw in Various Materials 183 

since it merely concerns the explanation of osmotic pressure, 
and is not essential to the integrity of our subject. Now a 
very remarkable and important point about osmotic pressures 
is this, that in general they are the same in amount as would 
be given by the respective substances if converted into gases 
at the same volume, temperature and pressure. This carries 
the implication that osmotic pressures and gas pressures, be- 
ing the same in amount, are the same in kind, the dis- 
solved substance being practically a gas, and it, not the liquid, 
exerting the pressure. But while this explanation is satisfactory 
for most of the phenomena, it meets with the physical difficulty 
that the closely packed water molecules must prevent that free- 
dom of back and forth movement upon which a gas pressure 
depends. Accordingly a second explanation has been given, 
really an old one revived, which finds the source of the pressure 
in an adhesive attraction between the molecules of the dissolved 
substance and those of the water, whereby the former draw all 
of the latter around them, and take more from the membrane 
(which easily recoups itself from the outside supply) ; and thus 
the solution swells and the pressure is obviously exerted by the 
substance and liquid in combination. Or, one can express the 
same thing by imagining that the molecules of the dissolved 
substance act like the micellae of the membrane and absorb 
water (from the latter) by imbibition, with only this difference 
that the adhesion between substance and water is stronger for 
all distances than the cohesion of the substance for itself. And 
still a third explanation is possible, namely, that the spaces 
between the suspended molecules of the dissolved substance act 
like excessively fine passages along which the water passes forcibly 
by an extremely refined capillarity, in which case the water, and 
not the substance, exerts the pressure. And if it seems that 
the correspondence between osmotic pressures and gas pressures 
must be conclusive for the first explanation against the others, it 
is to be said that this is not necessarily true, for the properties 



1 84 



The Living Plant 



of substances in solution and in the gaseous state are so closely 
and regularly interconnected, that the same mathematical rela- 
tions apply to them all. And as to which of the explanations is 
correct, the future must decide. 

The reader has now a sufficiency of data for understanding 
pretty fully the nature of osmotic absorption and pressure, which 
we may summarize here by aid of the accompanying diagram 

(figure 63). The dissolved sub- 
stance inside of a membrane is 
always tending to diffuse out- 
ward by the energy of its own 
diffusion pressure, which de- 
pends ultimately upon heat; 
and if the membrane be per- 
meable, then the substance dif- 
fuses into and beyond it r as it 
did from our molasses-holding 
osmoscope; but if semiperme- 
able then not. Meantime, 
whether because the interrupted 

FIG. oS.-Diagram to illustrate osmosis diffusion-preSSUre acts like gaS- 
through a permeable membrane*; the * 

symbols as in figures 58, 59. In case, pressure to Swell the interior 

however, the membrane is semiperme- .. . , , f n 

able the dissolved substance cannot es- liquid, Or beCailSC OI adhesive 

cape through it. attraction between substance 

and liquid, or because of capillary action between substance 
and liquid, the substance draws on the water supplied by the 
membrane, which yields it very easily so long as it can recoup 
itself freely from the outside supply. Thus the solution swells 
and exerts pressure until the power of the substance to with- 
draw water from the membrane exactly balances the resistance 
interposed to its expansion. And this is all true inside of the 
plant or the animal as well as outside thereof, whence we may 
now deduce another of our natural verities, to this effect, that 
wherever the conditions for osmotic absorption exist, the membrane 





How Plants Draw in Various Materials 185 

acts as a check, either partial or total, to the further diffusion of the 
dissolved substance while allowing the liquid itself to pass freely, as 
a result of which the dissolved substance, whether by gas-like ex- 
pansion or direct attraction, draws liquid through the membrane, 
swells, and exerts an osmotic pressure proportional to its strength. 

After this lengthy but needful discussion of physical principles, 
we turn to the actual osmotic phenomena displayed by plants, 
and here the reader who is skipping the hard parts must resume 
the narrative. The absorption of water by roots is the most 
important of these phenomena, but there are others of little less 
consequence. First among them is the maintenance of rigidity 
in very soft parts such as leaves, young steins and flowers. These 
parts consist mostly of water (fully 90 per cent), while the re- 
siduum of solid matter (about 10 per cent) is too small and un- 
substantial to supply rigid support. Even the moderately firm 
veins, as everyone knows, are quite unable to keep a wilted 
leaf from collapsing. But every young cell, soft and weak 
though it is, can absorb water powerfully through its semi- 
permeable protoplasmic membrane into its sugar-holding sap, 
and thus swell to turgescence, stretching the walls until they are 
tense, and the structure is stiff. Again, osmotic pressure supplies 
the energy by which young cells can expand their walls in growth, 
overcoming the resistance of older cells around them; by which 
buds or flowers can swell and unfold; by which young roots can 
force a way through hard soil and even destroy masonry and lift 
curbstones; and by which soft-bodied fungi can burst pavements. 
Osmotic pressure is the mechanical power used by those parts in 
effecting their work. 

Of minor osmotic phenomena in plants, some of them familiar 
in the household, there arc many. Thus, if one places dry sugar 
on fresh strawberries, pretty soon it becomes a syrup, and the 
berries look shrunken; evidently the sugar, moistened by con- 
tact with the berry, makes a dense solution which draws water 
from the cells. The collapse of berries from this cause is very 



1 86 The Living Plant 

evident when preserves are made with plenty of sugar, but 
fruits retain their shape in some of the processes where little or 
no sugar is used. Dry raisins and currants become plump when 
soaked, for their cells contain sugar though their protoplasm is 
dead; and the process is hastened by the heat of cooking. The 
crisping of celery or cucumbers when placed in water is a case of 
increased turgescence, the tense cells actually exploding, as it 
were, when crushed by the teeth. The reason, by the way, why 
the water must be cold for best crisping is this, that warmer 
water tends to drive out and replace the air of the intercellular 
passages, thus deadening the explosive action in which crisping 
consists. Moreover, the bursting of hard-skinned berries, like 
cranberries, when heated in water, though apparently an os- 
motic phenomenon, is primarily due to the swelling of the air 
confined by the skin, the same thing which occurs in apples when 
baking. A genuine osmotic bursting does, however, occur some- 
times in fruits, like plums and grapes, while still on the plant, 
because of a great absorption of water from the ducts by the 
sugar-ripe cells under action of heat on bright summer days; 
and the calyx of carnations sometimes bursts from the same 
cause when the temperature rises in the greenhouse. There is 
in tomatoes an osmotic disease, called (Edema, due to an over- 
absorption of water by soft cells, and the consequent formation 
of blistery swellings. The swelling of soaking seeds with a power 
sufficiently great to result in the bursting of strong vessels, is 
chiefly due to osmosis though it is partly imbibition, and the 
same is true of the forcible swelling of dried apples. Sugar and 
salt are common preservatives, the one of fruits and the other of 
meats, though neither is really poisonous to the germs and molds 
which cause decay, while the former is actually nutritive; but in 
strong solutions they act germicidally, because they withdraw 
so much water from the decay organisms as to render these 
inactive. Moreover, either of these substances, when eaten in 
more than moderate amount, causes thirst, which results from 



How Plants Draw in Various Materials 187 

their osmotic action in withdrawing water from the walls of the 
stomach, whose dry ness, from whatsoever cause, gives the thirst 
sensation. And there are doubtless other familiar osmotic 
phenomena which will occur to the ingenious reader, who can 
now have the pleasure of undertaking their explanation upon an 
osmotic basis. 

To complete our discussion of water absorption by plants, we 
must consider the case of dry tissues like wood. Dry wood, as 
everyone knows, absorbs water eagerly and powerfully, swelling 
considerably in the action. The conditions for osmosis are ab- 
sent, and all evidence goes to show that the absorption is due to 
imbibition into the solid cell-walls. This helps to explain a 
common phenomenon in connection with wood, its warping. 
When water is placed on one side of a dry board, the board warps 
away from the wet side, often with power enough to tear it from 
firmly-fixed fastenings; but if the supply of water be continued, 
the board later flattens out, and a measurement will show that the 
saturated board is considerably larger than when dry, precisely 
as a membrane is. Evidently, the water forcibly absorbed by 
imbibition upon one side forces apart the micellse and swells 
the wood on that side before it has time to reach the other, al- 
though, after the lapse of enough time, it penetrates to the other 
side, swells that, and thus straightens the board, as represented 
diagrammatically by a combination of the figures 59 and 64. It 
will here occur to the reader, incidentally, that boards often warp 
without access to water, and simply from the one-sided action of 
heat. The principle, nevertheless, is the same; even the dryest 
boards contain some water, the drying of which from one side 
allows the water remaining in the other to warp the board in the 
usual manner. Furthermore, the reader may recall that a board 
will warp crosswise but never lengthwise, which fact is correlated, 
obviously, with another well-known fact about wood, a fact 
of very great importance in building and carpentry, viz., that 
wood does not lengthen or shrink lengthwise as it does so freely 



1 88 



The Living Plant 



crosswise. The basis of this fact is not known, but I venture to 
suggest as a possible explanation that the sides of the cubical 
micellae facing towards the end of the wood (those towards and 
away from the reader in the sections of figures 59 and 64) have 
no attraction for water at all, and hence absorb none; and this 
view I propose that we hold as an hypothesis until it is disproven 




Fio. 04. A diagram illustrating the molecular basis of the warping of wood. It belongs 
between a and b of figure 59. 

or a better is offered. The supposition that micellar surfaces can 
exist without any attraction for water will help also to explain 
how cell-walls can be waterproof, as they actually are in cork 
and epidermis. 

A special form of imbibition by dry tissues is the absorption 
of water vapor from moist air, with its return thereto as the 
air becomes dry, a phenomenon called hygroscopicity. Fa- 
miliar examples occur in the softening and sagging of paper in 
damp weather, in the uncurling and curling of hair, in the move- 
ments of the wood of old furniture, giving rise to snappings and 
creakings which are oft of uncanny effect when heard in the 
stillness of night. Now, in essence, hygroscopic movement is the 
same thing as warping, the water being absorbed as a vapor 
instead of as liquid. Furthermore, if the tissues are made very 



How Plants Draw in Various Materials 189 

thin, this warping may be rapid enough to be seen by the eye, 
and forcible enough to exert a considerable pressure; and ad- 
vantage of these features is taken by plants, to produce, by aid 
of suitable mechanical arrangements, adaptive movements of 
various sorts. Of this nature are sundry hygroscopic movements 
described elsewhere in this book, the self-planting of some 
seeds; the creeping of some fruits by the twisting movements 
of hygroscopic awns; the opening and closing, with changes of 
weather, of most spore-cases and anthers; and the forcible shoot- 
ing of seeds by hygroscopically-bursting pods. Man has also 
taken advantage of this principle to construct instruments, 
called hygroscopes or hygrometers, for showing or measuring 
the amount of moisture contained in the air. By suitable mechani- 
cal arrangements the hygroscopically swelling or shrinking sub- 
stance may be made to twist a pointer over a graduated scale, 
to cause suitably-clad little persons to make their exits and en- 
trances to and from tiny houses, or to produce other visible 
results having appropriate significance. 

So much for the absorption of water; we turn now to absorp- 
tion of minerals, several kinds of which are needed for the various 
processes of metabolism inside of the plant. But the subject is 
comparatively simple. The plant can absorb only those minerals 
which exist in solution in the water of the soil, dissolved therein 
from the rocks or from various fertilizers added by man. And 
the minerals enter the plant with the water. In Water-plants, 
and the simpler sorts of the land, they enter mostly by diffusion 
from the outside supply, traveling everywhere through the water 
which saturates the plant. But in the higher plants they are 
swept in with the current through the hairs, cortex and ducts, 
from which they pass by diffusion to the places of use. It would 
seem at first sight that their passage through hairs and cortex 
would be forbidden by the semi-permeable protoplasmic mem- 
branes. But semi-permeability is wholly relative, and a given 
membrane which prevents the passage of the relatively large 



i go The Living Plant 

sugar molecules, may permit the passage of the much smaller 
mineral molecules. But aside from this, the evidence shows that 
in protoplasmic membranes another influence comes into play, 
and that the dissolved substance, in order to pass through such 
a membrane, must be soluble in the material composing it. 

There remains to be considered the absorption of gases, a matter 
of great importance because of the indispensable part played in the 

plant's economy by both car- 
| bon dioxide and oxygen, the 
great reservoir of which is 
[the air. The first requisite, 
of course, to gas absorption 
by the living cells, the most 
I of which lie deeply buried 
within the body of the plant, 
is some system whereby those 
gases can be conveyed from 
the atmosphere into their 
presence; and such a sys- 
tem, as the reader already 

Fiu. 05. A cluster of cells in a piece of pith, i Ipornprl in Phonfor TT i 
showing the intercellular air passages (in naS learned 111 L^aptCl 11, IS 

black). (Copied from a wall diagram by provided in the inter-cellular 

Frank and Tschirch.) r 

air passages, which are shown 

in a typical tissue, a bit of pith, in the accompanying picture (fig- 
ure 65). These passages do not exist in young tissue where new 
cells are in process of formation, as figures 53 and 139 C illustrate ; 
but as the young cubical cells grow larger, they tend to round off 
into spherical form, splitting in their mid-walls, first at the 
angles and then along the edges, until the final arrangement 
tends to approximate to that of the spaces and passages existing 
between balls in a pile. These passages once formed always 
persist, no matter what shapes the (Jells may assume; and there- 
fore they form a continuous system ramifying everywhere 
throughout the plant, as is represented diagrammatically in the 




How Plants Draw in Various Materials 191 

accompanying figure 66, A. Here, for simplicity, the passages, 
represented in black, are imagined to fall into one plane; and here 
also, by the way, a partial interruption in the system due to 
the presence of the longitudinally-running woody bundles is 
shown by the blank spaces. In young green tissues, as shown by 
the detailed diagram (figure 60, B), in which, as in C and /), the air 
passages are partially reduced to one plane, the passages open 
through the epidermis by the stomata, while on older stems, 
where a corky bark has formed, they open through the lenticels 
(figure 66, C), those corky wart-like excrescences prominent on all 
young stems, and consisting simply of open gashes in the bark, 
partially sealed in the winter by corky cortical cells. In young 
roots, however (figure 60, D), neither stomata nor lenticels are 
present, but the continuous epidermis and hairs are commonly 
and normally covered with films of water, through which the 
gases diffuse in solution from the air spaces in the soil to those in 
the root, and vice versa. 

Thus much for the aeration system, whereby every living cell 
of the plant is brought into communication with the external 
reservoir the air. But what is the power impelling the gases 
along these passages, which are often of great length, small size, 
and extreme irregularity? Plants possess no mechanism for the 
forcible indrawing and expulsion of the air en masse, such as 
animals have developed in their muscular chest-and-lung breath- 
ing arrangements. In some degree a movement of air through 
the inter-cellular system is promoted by the swaying of parts in 
the wind, and by the expansions and contractions of the air under 
varying temperature and barometric pressure; but such effects 
are insignificant. The primary cause of the gas movement is 
found in diffusion, that process, already described, whereby the 
molecules, driven by the energy of heat absorbed from the sur- 
roundings, tend ever to move forward from places of greater to 
places of lesser concentration, and therefore from places where 
they are being formed or released to places where they are not, 



C. Longitudinal section through half of stem at y. 




D. Longitudinal section through a portion of root at * 
FIG. 66. Generalized drawings illustrating the aeration system of the plant. 

192 



How Plants Draw in Various Materials 193 

and from places where they occur to places where they are being 
absorbed. Moreover, each kind of gas diffuses by itself, no mat- 
ter what others may be present, so that a gas in process of ab- 
sorption by a plant can move inward in a steady stream through 
another which is not being absorbed, and even against the op- 
posite stream of one in process of release. Thus, in photosynthe- 
sis, for example, a constant current of carbon dioxide diffuses 
into the leaf, through nitrogen which remains without move- 
ment, against a current of oxygen which is diffusing outward. It 
is a condition hard to imagine, it is true, but the facts declare it 
is so. The gases thus impelled along the passages by diffusion 
finally reach the living cells, and, being soluble in water, are 
dissolved by the moist surfaces, and then diffuse through walls 
and protoplasm to the places of use. And here I may add a sug- 
gestion, for the benefit of the reader versed in physics, that this 
movement of different gases in contrary directions along the same 
passages is explained much better by the old-fashioned idea that 
diffusion and gas pressure are caused by a mutual repulsion 
between the same kind of molecules than by the modern kinetic 
theory, which makes those phenomena the result of vibratory 
movements of the molecules; and moreover the very same con- 
ception explains perfectly how osmotic pressures and gas pres- 
sures can be identical in kind as well as in quantity. 

A very special case of absorption occurs in those plants which 
absorb organic food substances already made. Such plants, of 
which parasites are a good example, have the power of excreting 
from their absorbing parts those special enzymes, or ferments, 
which render soluble the organic materials they touch. The 
dissolved substance then enters the plant by diffusion from the 
place of high concentration outside to the places of use and low 
concentration inside, the intermicellar spaces, of course, being 
adjusted for the admission of these large molecules. The ab- 
sorption by pollen-tubes of tissues through which they pass; of 
humus by the fungi which live thereupon; and of the materials 



TQ4 The Living Plant 

dissolved from the bodies of insects by the pitcher-plants or other 
insectivora, is also of this character. In all of these cases the 
materials are not drawn in, as by osmosis, but are driven in by 
the energy of their own diffusion. 

In reviewing absorption by plants, the reader must be struck 
by the fact that the forces at work are chiefly molecular, and 
therefore slow and gradual, even though powerful in their action. 
Plants, as it were, arrange the conditions to permit the molec- 
ular forces to work for them. In this respect they stand in 
rather marked contrast to animals, which tend rather to make 
use of those larger or molar forces which permit greater rapidity 
and range of action. In this difference we have the explanation 
of the persistent placidity of plants in comparison with the 
abounding activity of animals. 

This chapter is already so long that it is only with reluctance 
that I add anything more; but there remain a few matters which 
must receive some discussion in this immediate connection. First, 
we must examine a little farther the arrangements for aeration in 
plants, especially under unusual conditions. Wherever particular 
need exists, there the inter-cellular system may become much 
larger, as occurs conspicuously in leaves, which, requiring a 
carbon dioxide supply for photosynthesis ten times or more 
greater than the oxygen supply they need for respiration, exhibit 
a far larger aeration system than any part of the plant needing 
only a respiration supply; and that is why leaves have the mark- 
edly spongy texture they so commonly exhibit. Again, there are 
plants of such habit that their roots (as in Marsh Plants), or 
even huge rootstocks (as in Water Lilies), lie deep under water 
and must be aerated in some way from the surface. In such cases 
the inter-cellular system is immensely developed, even to the 
formation of elaborate passages, in the parts which lead from the 
surface to the parts under water; and this is the reason for the 
soft, open, spongy texture of the petioles of Water Plants, and 
the pith of Rushes and Sedges, and it explains why some plants 



How Plants Draw in Various Materials 195 

can grow in a soil that has no aeration. And it is interesting to 
note, by the way, that many interesting accessory adaptions 
are displayed by these plants, of which one in particular is here 
apposite, viz., the walls of the air passages in these Water Plants 
are so modified chemically that water will not wet them, and 
therefore will not enter them by capillarity, on the principle dis- 
cussed earlier in this chapter. This is obviously an advanta- 
geous adaptation against the obstruction of these slender passages 
by water in case of sub-aqueous accident to the petioles or stems. 
In some other cases accessory aeration structures are developed 
which permit a shorter route from the air to the roots. Of this a 
conspicuous case has been claimed to exist in the great knees of 
the Bald Cypress of the Southern swamps, which rise above the 
water surface and contain an aeration system in connection with 
the roots; and other comparable cases are known. In some 
Water Plants, however, the aeration is of a simpler sort, con- 
sisting indeed of an absorption of air dissolved in the water, in 
precisely the manner used by the Fishes. In some kinds, for 
example some Eel-grasses, the leaves are so thin as to present 
a relatively great surface in proportion to the bulk of tissue to 
be aerated; while in others the leaves are cut to the finest divi- 
sions, presenting indeed a condition directly comparable physio- 
logically with the gills of the fish. This is the reason for the tissue- 
thin and thread-fine structure of practically all plants which live 
wholly under water. 

Finally we must give some further attention to the particular 
organs of absorption, the Roots. The structure of the young white 
tips has already been described except for one point, viz., the 
water-carrying ducts and the food-carrying sieve-tubes do not 
stand in-and-out from one another as in young stems, but alter- 
nately. In this arrangement lies an obvious adaptation, since 
it removes the sieve-tubes out of the path of the water from 
hair cells to ducts; and this conclusion receives some confirma- 
tion from the further fact that the arrangement is not main- 



The Living Plant 



tained in the older part of the root, where the entire anatomy is 
closely like that of the stem. Roots, however, have no nodes, 
nor regular places of origin of new roots, which, unlike branches, 
originate deep in the tissues, budding out as it were, from the 

fibro-vascular bundles (figure 67), 
and breaking their way (partially 
by the aid of enzymes) out 
through the cortex, at places de- 
termined by the stimulus of more 
abundant air, water, minerals, or 
space. This method of origin of 
side roots, by the way, stands in 
marked contrast with that of side 
stems, or branches, which always 
originate by a transformation of 
the cells of the cortex, as indi- 

Fio. 67. A cross section of a typical 

root, showing the way in which a side Cated in figure 137. IhUS, the 




NatUre and Devdopment of ways excessively irregular, al- 

though, on the other hand, differ- 

ent kinds of roots present comparatively little variation in 
structure or appearance, as indeed is to be expected from the 
comparatively uniform conditions under which most of them live. 
Typically, roots are much more slender than stems, and have 
their strengthening tissues condensed nearer the center, in obvi- 
ous correlation with the fact that they have no lateral strains 
to withstand, but only pulling strains exerted upon them by the 
stems for which they must provide a firm anchorage. Therefore, 
while stems approximate to hollow columns in construction, roots 
approximate rather to ropes or cables. Indeed, in many roots, 
one can trace a distinction between features connected with 
absorption arid others connected with anchorage of the stems; 
and the difference in some cases goes so far that one distinguishes 
between absorbing roots and anchorage roots, which often occupy 



How Plants Draw in Various Materials 197 

different positions or directions in the soil, the former seeking 
usually the dampest places, while the latter tend rather to pene- 
trate radiately from the stem into the earth. 

While absorption and anchorage are the typical functions of 
roots, occasionally they perform others quite different, as we 
have noticed already in the chapter on leaves and stems. Thus, 
they become modified, with appropriate anatomical changes, to 
swollen storage organs, in the Sweet Potato; to slender and 
toughened climbing organs in English Ivy and many tropical 
climbers; to tough pointed spines in some Palms; to slender 
penetrating haustoria or sucking organs in some parasites; to 
flat green photosynthetic organs in some tropical orchids; and to 
yet other structures of minor account. Thus roots, like stems 
and leaves, formed for one function can be modified greatly for 
the performance of others, illustrating once more Nature's won- 
derful capacity for ringing changes on her favorite ideas. 




CHAPTER VIII 

THE WAYS IN WHICH SUBSTANCES ARE TRANSPORTED 
THROUGH PLANTS AND FINALLY REMOVED THERE- 
FROM. 

Transfer) Transpiration, Excretion 

j]HE living plant, as the reader of the foregoing pages 
will surely agree, can be viewed as a kind of central 
station for the transformation of substance and energy, 
both of which forever are streaming into, passing 
through, and issuing forth from the plant, undergoing en route 
quite definite changes in correlation with adaptive results. These 
transformations we have already considered in our chapters upon 
Photosynthesis, Respiration, and Metabolism, while their Absorp- 
tion was the theme of the chapter just finished; but we still have to 
consider their passage through the plant and their final removal 
therefrom. These matters can be treated conveniently together 
as they are in this chapter, although, for a practical reason which 
will later appear, we may best reverse the natural order, and 
treat first the subject that logically should be last. 

The most abundant of the substances transferred and elimi- 
nated as well as absorbed, by plants, is water. Most people are 
aware in a general way that plants are forever giving off water 
as vapor to the air, although they have little idea of its amount. 
The fact can be demonstrated, by the way, very conclusively to 
the eye by placing a potted plant, of which pot and soil have 
first been enwrapped by a water-tight covering, in a glass case 
or bell-jar, after which, within a few minutes, there will collect 
on the glass a cloud of water-drops which can have come from 

198 



How Substances are Transported and Removed 199 

no other possible source than as vapor from the leaves. This is 
the source also of most of the moisture that collects upon win- 
dows near which house plants are grown, and likewise of the 
water-drops which gather, sometimes to annoying extent, on the 
glass faces of ferneries, though such water is commonly assumed 
to originate from evaporation out of the soil. This release of 
vapor from leaves or other green parts is a practically universal 
phenomenon in plants. It is called in physiology Transpiration; 
and I wish to warn the reader at this point, out of the depths of 
a considerable experience as a teacher, not to allow a mere re- 
semblance in words to create any confusion in his mind between 
this and the utterly unrelated process of Respiration. Transpira- 
tion is one of the great primal physiological facts about green 
plants, and it has, like Photosynthesis, this further distinction, 
that it is one of the very few processes of plants for which there 
is no equivalent in animals, the animal process of perspiration 
being utterly different both as to method and meaning. The 
reader should therefore incorporate into the visualized picture 
of the living plant now under construction in his imagination, 
the idea of a tenuous cloud of vapor rising forever from all its 
green parts. 

But no student of science, and therefore I hope not the reader, 
will rest content with the general fact that water is given off as 
vapor by plants, but will insist upon knowing the quantity. The 
most practicable and accurate of the several methods by which 
transpiration quantities may be determined lies in the use of the 
balance. If one takes an ordinary potted plant, Fuchsia, 
Hydrangea, Rubber Plant, or other, encloses soil and pot in a 
water-tight cover to prevent evaporation therefrom, then weighs 
the plant at intervals on an accurate balance, the comparative 
weights, aside from some minor, and largely self-compensating, 
errors arising from photosynthesis and respiration, must obvi- 
ously exhibit the exact transpiration from the leaves and the 
stems. Such experiments are frequently tried in botanical 



200 The Living Plant 

laboratories, and never without exciting an interested attention 
from all students, young or old. Some of the results are shown 
vividly in the accompanying photograph (figure 68), wherein the 
plant, with its pot and soil enclosed water-tight for this study, 




FIQ. 68, A potted Sunflower prepared for transpiration studies as described in the text. 
The measuring glasses show the number of cubic centimeters, and therefore of grams, 
of water transpired in twenty-four hours and in a week. In three and a half days the 
plant transpired a quantity of water equal to the capacity of the pot in which it is 
growing. 

is shown standing beside measuring glasses which display the vol- 
ume of its transpiration for a day and a week. The quantity of 
transpiration must necessarily depend on the size of the plant ; and 
in order to compensate this variable, and at the same time to 
permit a comparison between different plants, it is customary 



How Substances are Transported and Removed 201 

to express transpiration in standard units. For greenhouse 
plants, which have been the most carefully studied from this 
point of view, it has been found that while the transpiration in 
one hour from one square meter (roughly a square yard) of leaf 
ranges according to circumstances all the way from near nothing 
up almost to 300 grams (11 ounces), the generalized average, or 
conventional constant, is 50 grams per square meter (nearly 
2 ounces per square yard) per hour, i. e., 50 gm 2 h, by day and | of 
this quantity, 10 gm 2 h, by night, which equals 30 grams per square 
meter, 30 gm 2 h (an ounce per square yard) per hour, day and 
night together. Upon this basis, an average leaf during an ordi- 
nary summer season transpires an amount of water equal to its 
own area, and a centimeter (| of an inch) deep. These quantities 
are well worth remembering. 

The first sensation of the student as he really comprehends 
these data, especially whenever they are yielded by experiments 
of his own, is always one of surprise at the largeness of the quan- 
tity. It is, indeed, this copiousness of transpiration; rather than 
the existence of the process, which is the remarkable thing about 
it; and it helps to explain a number of more or less familiar 
phenomena. Thus, the rapidity with which leaves always wilt 
when cut from their stems, and the quickness and completeness 
with which plants can dry out the soil of their pots, are conse- 
quences of transpiration. In this way some plants can serve as 
good drainers of marshy soils. Thus Eucalyptus trees, especially 
active transpirers, have been used for this purpose in the Roman 
Campagna with such success that the marshes have become 
freed from the former scourge of malaria-carrying mosquitoes, 
and therefore habitable by man; while the malaria-repelling 
virtue often ascribed in this country to Sunflowers, which are 
sometimes planted around dwellings with this end in view, has 
the same genuine scientific basis. It is also transpiration condi- 
tions chiefly that determine which kinds of plants can be grown 


in dwellings as house plants. House plants are by no means the 



202 The Living Plant 

most attractive kinds there are, but are the most attractive that 
can withstand the dryness that prevails in our houses in winter, 
a dryness that is due not so much to the heat of the house as to 
the fact that the general atmosphere in the winter has a very low 
content of water vapor. A house plant in fact is one whose 
transpiration in that dry heat is no greater than can be com- 
pletely compensated by the absorption and conduction of water 
from the soil. And this relation of transpiration to conduction 
explains another notable phenomenon in plant nature, namely 
the limitation in the height of trees, which in general are just so 
high as the water can be conducted in sufficient abundance to 
supply the transpiration from the foliage. When that height is 
reached the tree can still spread out laterally, which explains the 
flat tops of the largest Elms, Maples, Oaks and others, and of 
many forest trees when seen from mountain tops. A transpira- 
tion effect of a very different sort is displayed by a good many 
plants in the early spring. It is a fact that roots absorb water 
very slowly when chilled, and if they are kept for a time at a low 
temperature, while leaves and stems are exposed to conditions 
favorable for transpiration, as is effected quite easily by experi- 
ment, the plants will wilt very rapidly. These very conditions 
are often supplied naturally in the spring, for if the soil remains 
frozen or very cold after warm bright days have forced out the 
leaves, or if a cold spell that chills the soil is followed abruptly 
by very warm bright windy days, then the young leaves transpire 
so much faster than the water can be supplied by the roots, that 
they become dry-blasted as if by a frost, to which latter cause, 
indeed, this effect is commonly but mistakenly ascribed. This is 
the explanation also of the fatal browning of the leaves of many 
ornamental evergreens, whose leaves are awakened to active 
transpiration before the roots can supply the water they need; 
and it is, indeed, a chief cause of winter-killing generally. And 
finally, as to transpiration effects, there is one more way in 
which this process exerts a very remarkable influence upon 



How Substances are Transported and Removed 203 

plants; for the necessity that it be regulated and minimized in 
places where water is habitually scanty, as occurs conspicuously 
in deserts, has resulted in the development of protective adapta- 
tions which, as the weird aspect of desert plants abundantly 
attests, affect the forms, sizes, and other structural features of 
plants more profoundly than does any other influence whatsoever 




FIG. 69. A transpirograph in action. The loss of a gram of water from the plant permits 
that end of the balance to rise arid close an electric circuit ; this acts, through an electro- 
magnet, to force a pen against a revolving time-drum (seen on the loft of the stand), 
and at the same time to drop a spherical gram weight from a cylindrical reservoir 
into the box under the scale pan, which is thus depressed, again breaking the circuit. 
Thus a record is made on the time-drum at each moment when the plant has lost a 
gram of water. 

excepting only Photosynthesis. But this subject belongs really 
with a later chapter (on Protection), where it will be treated in 
detail. 

The results of all experiments on transpiration show remarkable 
variations in its amount; but it soon becomes evident that such 
variations are correlated closely with changes in external condi- 
tions. This can be tested by weighing the plants while kept 
under somewhat extreme conditions of heat or cold, humidity or 
dryness, light or darkness; and the results are all the clearer if one 
makes use of some form of self-recording instrument, one of 



204 The Living Plant 

which, called a Transpirograph (a little thing of my own, by the 
way) is shown in operation in the accompanying photograph 
(figure 69). By its use the plant is made to write, precisely and 
continuously for days together, a record of its own transpiration. 
Further, there also exist instruments, invented long ago for use 
in meteorological stations, which write continuous records of 
the very conditions that affect transpiration, viz., tempera- 
ture and humidity, while light is recorded by a special method. 
When the cotemporaneous graphs of transpiration and the 
external conditions are plotted together upon the same sheet, as 
in case of the accompanying graph (figure 70), the relation be- 
tween process and influencing factors is displayed in a way 
which leaves little to be desired in the direction of exact and ex- 
pressive exhibition of the relation between this physiological 
process and the external conditions. Indeed, I am accustomed 
to use this study with my own students as an example of a well- 
nigh ideal piece of physiological method, whereby Nature is 
compelled not only to display, but even to write down, for the 
edification of man, the tale of her own operations. I often recall 
with delight the remark once made by an eminent literateur who 
happened to visit my laboratory at a time when this experiment 
was in progress. As soon as he had grasped the full scope of the 
matter, he turned away with this comment, "Well, I don't see 
what there is left for Nature to do but lay down and holler." In 
these words he expressed very well both the aim and the joy of 
scientific investigation, which after all is a kind of great game 
where one matches wits against Nature, and generally loses, but 
now and then wins and gathers the stakes, which consist in a 
share of her jealously-kept secrets. 

But to return to our experiments on the effects of external 
conditions upon transpiration, they show these results. Heat 
increases, and cold lessens it. Heat, indeed, may hasten tran- 
spiration to such a degree that water is lost from the leaves much 
faster than the roots can absorb it or the stems conduct it, in 




205 



206 The Living Plant 

which case a wilting results even though water is plenty in the 
soil ; but plants thus wilted can quickly recover when the weather 
grows cooler, for then the absorption and conduction catch up, 
so to speak, arid again fill the leaf. Light increases, and darkness 
lessens it. This harmonizes with our transpiration constants, 
which showed that in general the process is five times more active 
in daylight than at night; and it explains why plants that wilt in 
the day recover at night. Dryness increases, and humidity lessens 
it. This is the reason why most kinds of plants will not live in our 
houses, the air of which is so dry that the leaves lose their water 
much faster than roots and stems can supply it, no matter how 
plenty in the soil. It explains, too, why leaves never wilt in the 
weather called muggy, no matter how hot, and also why leaves 
that are wilted recover when sprayed, even though experiment 
proves that none of the spray is absorbed. As to other external 
climatic conditions, their influence is slight, except in the case of 
the wind, which always promotes it. Thus it is evident that in 
general transpiration is promoted by the very same factors which 
favor evaporation, though later studies have shown that the 
parallel does not hold true in detail. 

We must now consider the structural basis of transpiration, 
with which, however, the reader already has incidentally made 
some acquaintance. If he will recall his knowledge of the cellular 
structure of the leaf, refreshing his memory, perhaps, by another 
inspection of figure 2, Plate I, /?, and figure 54, B, it will be clear 
that every cell borders, for purposes of respiration and photosyn- 
thesis, upon the inter-cellular air-system, which ramifies through- 
out the leaf and opens to the outside world through the stomata, 
the little slit-like openings through the otherwise continuous 
epidermis. Now these cells are all gorged with water, which 
saturates their walls; and where these border on the air spaces 
the water necessarily evaporates. The vapor thus formed satu- 
rates the air inside of the leaf, and is then moved by the force of 
its own diffusion along the passages and through the stomata to 



How Substances are Transported and Removed 207 

the relatively dry atmosphere outside. Such is the structural 
and physical basis of transpiration, and it explains perfectly why 
heat, which is an evaporation accelerator, and dryness and winds, 
which are diffusion promoters, increase the process. 

But though such is its basis, transpiration is really not so 
simple as this, for it is influenced much by another condition, 
and that is the number and size of the stomata. As to their num- 
ber, that varies immensely with different kinds of plants, there 
being none at all on the upper surface of a good many leaves, 
while on lower surfaces they vary from a few up to near 500 to 
every square millimeter (one-twenty-fifth of an inch), with a 
conventional mean at 100; and this equals no less than 100 mil- 
lions to the square meter (yard), which is another of our con- 
ventional constants. And it is worth while to add that when all 
of the stomata are open their widest, about one-hundredth of the 
whole area of the leaf is exposed. As to the size of the stomata, 
that not only varies with the kinds, but in each kind is highly 
variable, since they open and close, from near a circle through a 
narrowing oval to a slit and perhaps no passage at all, by the 
movements of two bordering cells called guard cells. These 
guard cells, as shown by the typical example pictured herewith 
(figure 71), are of aspect distinctive and unmistakable, with 
little resemblance to others of the epidermis. They are usually 
somewhat kidney-shaped, forming together two halves of an 
elongated oval, and they contain chlorophyll. Their construction 
is such, as figure 71, lower, illustrates, that the natural spring of 
their walls tends to bring them together and close up the stomatal 
slit; but the development of osmotic turgescence in their cavities 
rounds them out so that they separate, thus opening the slit. 
Now this turgescence of the guard cells is influenced much by the 
quantity of water contained in the leaf, rising and falling there- 
with, so that when water is plenty the stomata tend to be open, 
but when it is scarce they tend to be closed. Thus it seems as if 
the guard cells ought to act adaptively as regulators of transpira- 



208 



The Living Plant 



tion, keeping it down to safe limits when water is scanty, but 
allowing full play when water is plenty. The turgescence of the 
guard cells, however, is influenced also in another way; for 

they (and they only of epidermal 
cells), contain chlorophyll, which 
has to make sugar in light and 
thus increase their turgescence 
and cause them to open the sto- 
mata. This arrangement would 
explain to perfection why light 
increases transpiration so greatly 
quite apart from any accompany- 
ing heat, while a definite ecologi- 
cal advantage seems equally 
clear, viz., it should ensure open- 
ing of the stomata at those times 
when the demand for carbon di- 
oxide is the greatest, and allow 
them to close with the lessening 
of this need. From the structure 
of the guard cells, therefore, we 
should expect them to serve 
as automatic valves, regulating 
FIG. 7i. Typical guard ceils, with a transpiration adaptively to the 

stoma between them, highly magnified, p Y f prn ol oonrlitioTm- and thim 

in surface view and cross section. The exT ^ rnai COnaillOnS, ana inus 

lower figure shows diagrammatically in fa e y have USUally been regarded 
cross section the method by which the t . 

turgescent rounding of their cavities by botanists. But this COncep- 

opens the stoma, the dotted walls ,. , , , , , 

showing the closed, and the unshaded 0n HaS not been Sustained by 

walls the open, position. (The upper i cfiidipq whlVh hflVP <*hnwn 

figures reduced from a wall-chart by iaier STiUCUes > Wmcn nave SnOWn 

L. Kny and the lower from a much- so mu ch irregularity, and CVCU 
copied diagram by Schwendener.) ^ ' 

anomaly, in their action that we 

have to remain in doubt until further researches shall give us 
the truth. Meantime we can only consider that any regulatory 
action they may have is clumsy at the best. 




How Substances are Transported and Removed 209 

Such are the principal facts as to transpiration, and they bring 
us to the problem of its physiological meaning, upon which also 
there is uncertainty. The older explanation argued thus: 
plants need in all parts, and especially their leaves, certain 
minerals from the soil: their only possible method, apparently, 
of raising these minerals to the places of use consists in absorbing 
and transferring them in water, and evaporating the latter to 
leave them behind : some of the minerals are so scarce that plants 
hardly ever can get as much as they need: the more copious the 
transpiration the more minerals are raised; presumably, there- 
fore, transpiration is the mineral-raising process and is the more 
efficient the more copious it is. On this assumption, plants would 
be expected to develop adaptations for promoting transpiration, 
and a great many such have actually been claimed to exist, as 
will presently appear. A second explanation argues thus: the 
stomata exist primarily for admission of carbon dioxide needed 
in photosynthesis (they occur, in general, only in green tissues) : 
when open for this purpose, evaporation and diffusion of water 
will necessarily take place from the saturated cell-walls of the 
interior of the leaf as a purely physical operation which the plant 
has no power to prevent: presumably, therefore, transpiration is 
merely an incidental physical accompaniment of photosynthesis, 
a kind of necessary evil, as it were. Upon this explanation 
adaptations would be expected for its prevention, especially of a 
kind which would not interfere with photosynthesis; and of these 
a good many have been described, as we shall note in the follow- 
ing chapter. This explanation accounts best for most of the 
phenomena, and is the one that is generally accepted at present. 
A third explanation argues thus: when full sunlight falls on a 
leaf, it beats thereon with an energy overwhelmingly greater 
than the leaf can employ in its work (for it actually uses no more 
than some three per cent): this energy, both light and heat, 
would work disaster to the living protoplasm unless dissipated 
in some manner: evaporation is a highly effective method of 



210 The Living Plant 

energy-dissipation: presumably, therefore, transpiration is an 
adaption to protection against injury from the over-plentiful 
energy of sunlight. Each of these explanations has its merits and 
its difficulties, and no one alone is sufficient. Probably the truth 
will be found to involve some participation of all three; transpira- 
tion may be fundamentally a process which the plant cannot 
prevent, but that is no reason why the plant cannot employ 
it, and even develop it highly, as an easy method of raising its 
requisite minerals, and a convenient means for the dissipation of 
superfluous energy. But this question, too, is one of the many 
whose solution lies with the future. 

Transpiration, however, is not the sole method by which water 
is removed from the plant. Everybody has noticed the clear 
shining drops which bejewel the margins of Grape leaves on 
mornings that follow hot days and cool nights; these drops are 
commonly thought to be dew but are not. They show very 
strikingly also on young plants of Nasturtium and seedlings of 
Grasses, where they can be made to appear whenever desired, 
simply by covering the actively-transpiring plants for a few 
minutes by a cooled, darkened, or dampened bell-jar. In a great 
many other plants, too, the drops appear and are mistaken for 
dew. The slender wet streaks often seen on the leaves of the 
Cannas just after sundown, come from similar marginal drops; 
and a tropical plant is said to exist from which water is projected 
in a very fine jet. In all of these cases the water is known to 
come from inside the plant, and the process, known physiologic- 
ally as guttatiouj is a result of the following conditions. On very 
warm days the vigorous transpiration is accompanied by an 
equally energetic absorption and transfer, but the comparatively 
sudden check to transpiration caused by the cool of the evening 
does not at once affect the absorption; therefore water continues 
to be forced into the stems and leaves to an extent which might 
prove a serious detriment were it not for an avenue of escape pro- 
vided by openings existing in the ends of the veins, for it is here 



How Substances are Transported and Removed 211 

that the water-drops always appear. Guttation, therefore, is a 
kind of a safety-device for the plant even if transpiration is not. 
Furthermore, it happens at times that roots keep their vitality 
long after the stems have died, and continue to force up water 
which can find an outlet only through rifts that it makes in the 
withering stems. Besides, in cold weather all stems tend of 
course to contract, thus squeezing from such rifts any over- 
abundant water they may happen to contain. When water from 
either of these sources is forced out in cold weather, it freezes in 
lines, which soon become flat plates as more and more issues 
from the stem, pushing the already formed ice before it; and this 
is the origin of the ice crystals or shells, often of great beauty and 
commonly mistaken for "frost," which are seen on the stems of 
some plants in the early part of the winter.* 

If I seem to have dwelt over-long on this matter of water- 
removal from the plant, I claim in explanation that the process, 
because of the profundity of its effect upon plant-structure and 
habit, is worth all the space I have taken; and the later chapter 
on Protection will help to support this conclusion. But now we 
are ready to proceed to the topics remaining, of which the re- 
moval or excretion of substances other than water comes naturally 
next. These excretions belong to four different classes. First, of 
course, are the gases, for oxygen is an excretion in photosynthesis, 
and carbon dioxide in respiration. But the subject is simple, for 
they pass off by diffusion, either through stomata and lenticels 
of leaves and stems, or in solution through the wet epidermis of 

* A conspicuous case occurs in Helianthemum canadense, commonly called Frost- 
weed, which is described in Gray's Manual of Botany thus: "Late in autumn crystals 
of ice shoot from the cracked bark at the base of this and the next species, whence 
the popular name." Another, and even more striking, example is the Dittany 
(Cunila Mariana, or origanoides), in which the ice-forming habit has thus been de- 
scribed: "Our Cunila has attached to the stem a shell-work of ice, of a pearly white- 
ness, beautifully striated, sometimes, like a series of shells one in another at others 
curved round on either side of them like an open, polished, bivalve; then, in others, 
again, curled over in every variety of form, like the petals of a tulip." (J. Stauffer, 
quoted in the Botanical Gazette, XIX, 1894, 326.) 



212 The Living Plant 

the roots. Second, are various minerals, which in part are useless 
materials absorbed along with the useful kinds, and in part are 
by-products of chemical changes inside of the plant. For their 
removal plants have no regular excretory system as animals 
have, though a partial substitute exists in the fall of the leaves 
and the bark, which thus remove crystalline matters they con- 
tain. Other minerals are left behind as crystals in the old dead 
cells when the living protoplasm advances into the new ones it 
forever is building (compare figure 41). Third, are the root- 
poisons, little known to us yet and even by some experts not be- 
lieved to exist. They appear to be highly complex organic sub- 
stances, slow of diffusion and drainage, and poisonous to the roots 
which produce them though not necessarily to different kinds; 
and this fact gives a new explanation of the advantage of rotation 
of crops and of letting a soil lie fallow. Fourth, is extra-floral 
nectar, apparently identical in composition and mode of forma- 
tion with the nectar of flowers, which performs the invaluable 
service of attracting cross-pollinating insects, as later we shall 
note in detail. The extra-floral nectaries are very tiny structures, 
sometimes marked by blotches of color, occurring commonly at, or 
near, the bases of leaves in young plants (e. g. in some Ferns, 
Horse Beans, Castor Beans and others), or with the spines (in 
Cactus), and elsewhere. They have been supposed to attract 
small ants which may perform some ecological service; but the 
evidence thereon is so unsatisfactory that it seems best to place 
this nectar for the present among the excretions, though surely 
it is a puzzling sort. 

So, and by such means, are substances removed from plants, 
The reader knows also in what ways they are absorbed. Between 
absorption and removal they have to be transported, often for 
very long distances; and this is the matter which next needs 
attention. 

The principal substance to be transported is water, of which 
transpiration demands so great a supply that it has to be moved 



How Substances are Transported and Removed 213 

in a copious and continuous current through the plant. This 
involves of course a highly efficient water-carrying mechanism, 
which we should first consider. The principal feature thereof is 
the ducts, which are tubes, beginning near the tips of the roots 
(figure 53) and running in bundles throughout the length of the 
stem to the leaves, as our earlier generalization of the system 
so clearly illustrates (figure 54, A) ; and here they end in little areas 
of green tissue, as we have noted already in the description of the 
leaf. Structurally, the individual ducts are short, but the end of 
each one lies against the end of another with only a thin partition 
between; and therefore the practical effect is that of a continuous 
tube with occasional thin cross partitions. When roots and stems 
are young and flexible, the soft walls of the ducts are supported 
inside by ringed or spiral thickenings, which keep the cavities 
open when the young roots or stems become sharply bent back 
by accident, and also against the turgescent pressure of neighbor- 
ing cells. The ducts formed later, however, when the tissues are 
thicker and harder, have not the spirals, but stiff bands or a fret 
work, or even a uniform thickening, pierced by thin areas for 
the escape of some water to the neighboring tissues. These dis- 
tinctive features of ducts are very well shown in the picture 
given herewith (figure 72; also 54, C). 

We turn now to the study of the transfer of water through the 
plant, or, as it may also be expressed, the forces impelling the 
ascent of sap. Transpiration makes very great demands for a 
water supply, especially in lofty and broad-leaved trees, and in 
weather that is bright, dry, and windy. By what forces is so 
weighty a volume of water raised so quickly to a height so great? 
Recently I had occasion to calculate the work done in a day in 
transferring the water from roots to leaves in one of the largest 
kind of trees, and I found it was just about equal to that which 
would be done by a man in carrying 500 large pailfuls of water 
up a ten-foot flight of stairs within ten hours. This is nearly a 
pailful a minute for ten hours without cessation, my figures being 



214 



The Living Plant 



expressed in this form in order to bring the matter home to my 
students. Now, strangely enough, the botanists are not yet 
agreed either as to the source of the energy or the precise physical 
method by which this considerable work is accomplished; and in 
default of precise information I can only present to the reader a 
synopsis of such data as we possess, along with some comments 
on their probable bearing. And here follow the principal explana- 
tions which have been offered for the physics of sap ascent. 




FIG. 72. A generalized drawing of the tissues of a typical stem, showing the water- 
carrying ducts (the three larger tubes), and a food-carrying sieve-tube (the single 
dot-lined tube), with the associated tissues. (Copied from Kerner's Pflanzenlchen.} 

1. Root pressure. In the preceding chapter it was shown that 
roots absorb water osmotically and forcibly start it up the ducts. 
But this pressure, which, in some greenhouse plants has been 
found sufficient to raise water 40 to 50 feet, and in trees up to 
80 feet, is wholly insufficient to explain the ascent when trees 
reach 400 feet, as they do in some kinds of Australian Eucalyptus; 
and therefore this cannot be the explanation. 



How Substances are Transported and Removed 215 

2. Atmospheric pressure. This will suffice, when the suitable 
conditions are provided, as they are in a pump, to raise water 
some 32 feet, but no more; in the plant, however, the requisite 
conditions are wanting, while this height is obviously quite in- 
adequate. Therefore this cannot be the explanation. 

3. Capillarity. This is the power, as the reader will recall, 
by which water, driven by its own internal molecular energy, 
rises in small tubes, the higher the smaller the tube. But even 
the slenderest ducts known to occur in plants are not small 
enough to raise the water more than a few feet even if all the other 
conditions were most favorable, which indeed they are not. 
Therefore this cannot be the explanation. 

4. Imbibition. This was the favorite theory of the great 
botanist Sachs, who defended it to the end of his life. He con- 
ceived of the wall-system of the plant as a kind of gigantic con- 
tinuous membrane, extending all the way from the root hairs to 
the cells of the leaf; into this membrane, by forces and method 
already considered, water was absorbed by imbibition, and raised 
by the same energy, to be finally removed by evaporation at 
the leaf-cells. The theory is simple and plausible, but is shattered 
by one fatal fact, viz. it requires that the transpiration stream 
shall move in the walls of the ducts, not their cavities (which 
Sachs took simply for reservoirs), whereas experiment proves 
beyond question that the water does move in the cavities. There- 
fore this cannot be the explanation. 

5. Propulsion. This theory maintains that the water is 
forced or propelled upwards by some action of the living cells 
distributed along the course of the ducts, each living cell being 
supposed to draw water from a lower duct and force it out into a 
higher. It really is an extension of root pressure to the whole 
stem, the living cells passing water from one duct to another 
precisely as the root hairs and cortex pass it from the soil into the 
ducts, and by the very same physical power and method, which 
is still unknown in detail. It differs from the preceding explana- 



216 The Living Plant 

tions in this, that it involves the activity of cells which are alive; 
and herein also it meets its greatest difficulty, because, accord- 
ing to some experimenters, when the living cells are killed by 
suitable methods, the water continues to ascend, at least for 
some time. Therefore, they say, this cannot be the explanation. 
But others are not convinced that the cells are really all killed in 
these experiments, and hold that this explanation is substantially 
correct, 

6. Traction. This, the most recent explanation, has been 
worked out by a botanist, Dixon, and a physicist, Joly, working 
in collaboration, and is often known by their name. It maintains, 
in brief, that water in very thin threads holds together, by the 
force of its own internal cohesion, with a tenacity sufficient to make 
it as strong as a solid fiber or wire; wherefore the thin threads 
of water in the ducts can actually sustain their own weight for 
a length as great as the height of the tallest trees. These threads 
being practically continuous from the tips of the roots to the 
cells of the leaves, hang, as it were, from the leaf-cells, into which 
they can be lifted by any power that can remove the water from 
those cells. This power is supplied by the energy of evaporation 
in transpiration, which latter process, therefore, lifts or drags 
the water threads up the ducts much as a man on a roof would 
pull up a rope from the ground. On this view the energy which 
raises the water in the tree is the same which lifts it to the clouds. 
This theory finds its chief difficulty in the lack of complete demon- 
stration that the water can thus cling together in threads of such 
great length, and it has not been universally accepted. 

It sometimes appears as if the extent of our knowledge of any 
subject were inversely proportional to its importance. At all 
events we found this to be true of the structure of protoplasm, 
and it also seems true of this subject of sap ascent. And at 
present there is a pause in the advance of our knowledge thereof. 
With this subject, as with others, we find out everything that 
existent methods of investigation can yield, then turn for a time 



How Substances are Transported and Removed 217 

to other matters. Presently, however, somebody, working per- 
haps in a quite different field, chances upon some new method 
that happens to be applicable to this subject, to which students 
then turn once more, and make another long step in advance. 
The very fact that all knowledge thus grows by appreciable 
stages makes it all the more interesting to follow; and the watch- 
ing for such new knowledge, and the grasping it when it appears, 
constitute the principal charm of the scientific life. 

There remains but one other point in connection with the 
transfer of water. The current must supply not only the tran- 
spiration loss, but all the working needs, chemical, osmotic and 
other, of the various tissues besides. This matter, however, is 
simple, for all kinds of ducts possess plenty of thin places through 
which the water can pass outward, after which, by imbibition 
and osmosis, it gradually penetrates from cell to cell throughout 
all of the tissues that need it. And with the water in this way go 
the various minerals in solution, which explains their transporta- 
tion, as well as their absorption, by the plant. 

From the transport of water and minerals we turn to that of 
the various food-substances made in the plant, a subject known 
in plant physiology as translocation. The subject is comparatively 
simple. In the first place such substances travel invariably in 
solution; and substances which are not soluble in water never 
move from their places of formation. The very physical nature 
of some substances, e. g. the sugars, makes them naturally soluble, 
but others, viz. starches, oils, cellulose, and most proteins, are for 
the same reason insoluble. In such cases solubility is obtained, 
for purposes of translocation, by their conversion (or hydrolysis) 
into closely-related substances which are soluble, thus starch 
and cellulose into sugar, oils into fatty acids, insoluble proteins 
into peptones. These changes are effected by those remarkable 
substances called enzymes, whose method of action we have con- 
sidered in the chapter on Metabolism. The enzymes are widely 
scattered through plants, and some of them are identical with the 



218 The Living Plant 

digestive juices (diastase, pepsin) found in the alimentary system 
of animals; for the solution or hydrolysis of insoluble foods by 
enzymes constitutes digestion in plants just as truly as in animals. 
This digested material is then in suitable condition for transporta- 
tion, which takes place in two ways. First, it may be carried 
with an onward-moving water current, as happens with the sap 
in the spring (witness the Sugar Maple), when the food stored 
for the winter in the roots or lower trunk of the tree diffuses from 
the storage cells into the sap current and rises therewith. Sec- 
ond, it may travel by diffusion alone, for a substance dissolved 
in water is in perfect physical condition for diffusion, that 
is, has the power and the tendency to move outward and on- 
ward, by its own diffusive energy, from places of greater to 
places of lesser concentration until equilibrium is established. 
When, furthermore, the substance is being produced at one place, 
as occurs with sugar in the leaves during photosynthesis, and is 
being removed in another, as occurs in places of storage where it 
is converted into insoluble starch, then a steady diffusion current 
is established between the place of production and the place of 
use. And it is by such diffusion currents that most of the trans- 
location of food-substances through the plant is effected, though 
it is to be remembered that diffusion alone, from its very nature, 
can never completely empty a part. This explains why some 
sugar and other food materials remain in autumn leaves when 
they fall. 

This translocatory diffusion proceeds in part from cell to cell 
through the walls, the protoplasmic linings thereof being adjusted 
(by appropriate chemical modification or intermicellar spacing, 
as noted earlier under Absorption) to permit the passage of the 
molecules of the substance; and, given time enough, there is no 
limit to the distance that substances may thus pass in solution. 
Obviously, however, such translocation through long distances 
must be greatly facilitated if long tubes replace the short cells; 
and such a system is actually found in the elongated sieve-tubes 



How Substances are Transported and Removed 219 

which are very well illustrated in our figure 72. These sieve-tubes 
accompany the ducts all through the plant from root-tips to 
stem-tips and leaf-cells, as our generalized plant illustrates so 
clearly (figure 54), thus forming a part of the same fibro- vascular 
bundles. But sieve-tubes are more slender than ducts, and unlike 
them have thin soft walls, and a continuous lining of protoplasm; 
while the occasional cross partitions, thicker than the walls, are 
perforated by openings in a way which has given these structures 
their name (figure 54, C). The presence of this protoplasmic 
lining in the sieve-tubes when diffusion alone does not require its 
presence at all, suggests that it plays some part in helping to force 
substances along the tubes, perhaps in a manner analogous to the 
way in which the food is moved along the intestines of animals; 
but no such action has been proven. Doubtless the movement is 
aided materially by the swaying of branches in the wind, and, 
when it is downwards, by gravitation; but these influences are 
obviously both incidental and irregular, and diffusion is the only 
motive force in translocation that we surely know. The reader, 
therefore, must visualize this process as one of constant diffusion 
along the sieve-tubes. It is not an onward movement of the 
solution they contain, but a movement of the sugar and other dis- 
solved substances through water that is standing still, a process 
in great contrast with the onward rush of sugar-carrying sap in 
the spring. The method of this diffusion, by the way, is illus- 
trated diagrammatically in figure 6. 

The sieve-tubes, in which translocation of food principally 
proceeds, lie in the inner bark of woody plants, down through 
which, accordingly, all summer long, there is a constant move- 
ment of food-substances towards the roots or other underground 
parts devoted to winter storage. That this is really the path is 
easily proven by experiment, such for instance as removing a 
narrow ring of the bark, or constricting it by a metal ring. This 
often happens by accident in Botanical Gardens where the en- 
circling wires which support the labels are left too tight. In all 



220 The Living Plant 

such cases the obstruction in the bark causes an accumulation 
of the food just above, with a resultant swelling of the tissues 
that often is very prominent. The same thing happens also 
naturally where a twining stem, such as that of a Bittersweet, 
tightly constricts a growing tree, in which cases the swelling stem 
always shows a very much greater enlargement above than below 
the vine. 

Such is the method whereby food materials are transported 
from their places of formation to the places of storage end use. 
The same general method explains the transport and accumula- 
tion of all those special substances, usually of definite and adapt- 
ive functions, which we call secretions, the volatile oils, nectar, 
some coloring matters, and others which have been considered 
in the chapter on Metabolism. 

This is really the place to bring this particular chapter to a 
natural conclusion ; and it is truly a pity that it cannot be done. 
For somewhere in the book we have to consider the prominent 
subject of the cellular anatomy of stems, and this is the most 
suitable place. However, the matter is not indispensable to a 
clear understanding of the chapters that follow, and therefore 
the reader may skip the remainder of this chapter if he wishes. 
And if the said reader should ask why I do not skip it myself, I 
would answer that the integrity of my subject requires its pres- 
ence. For with regard to this book I feel with Nehemiah Grew, 
who wrote more than two centuries ago in the dedication to his 
great work on the Anatomy of Plants, "Not I, but Nature 
speaketh these things." 

If, accordingly, in pursuit of a knowledge of the anatomy of 
stems, one cuts with a sharp knife a clean section across any 
young stem, he can always discover the ends of the fiber-like veins 
distributed in a uniform ground-work of tissue. And if, further- 
more, he makes a thin section from a typical young stem, such as 
Castor Bean, and magnifies it moderately, he will have before 
him such an appearance as is pictured herewith (figure 73). 




How Substances are Transported and Removed 221 

while a typical stem is shown generalized in our later figure 139 B. 
Among the many cellular elements in the symmetrical, almost 
geometrical structure thus displayed, it is easy to identify the 
bundles of ducts from their relatively large size and their obvious 
resemblance to the cut ends of 
round tubes. Associated with 
the ducts, and a little way re- 
moved towards the outside of 
the stem, lie clusters of smaller, 
thinner-walled, and more angu- 
lar cells, which are also the cut 
ends of long tubes, the food- 
carrying sieve-tubes ; while be- 
tween sieve-tubes and ducts lie 
two or three layers of small 
squarish cells presenting an 
aspect which later the reader Flo 73 ._ Cross Scct i n of a young stem of 

Will learn to associate With the Castor Bean, magnified nlxiut twenty 

times. (Copied, reduced, from a drawing 

growth, for they are the Cam- by H. O. Hanson, in Curtis' Nature and 
7 . .. , , /. T Development of Plants.) 

mum cells which form new ducts 

and sieve-tubes as long as the plant lives. Ducts, sieve-tubes and 
cambium, to which often are added strengthening fibers, grow 
all or a part of them together in bundles, forming fibro-vascular 
bundles which are identical with the veins, both the kind that 
can be seen in young translucent stems, and also those familiar 
in leaves. The bundles begin, as our generalized picture of the 
conducting system illustrates (figure 54), near the ends of the 
roots, where they consist of a few ducts and sieve-tubes only; 
farther back they acquire cambium and fibers and enlarge greatly 
in size; in the stem they branch at the nodes and run out to the 
leaves, when they fringe away gradually to the veinlets, each of 
which ends as a single duct and sieve-tube in the midst of one of 
the ultimate areas of green tissue. 
The fibro-vascular bundles have not only this definite com- 



222 The Living Plant 

position, but a definite arrangement in the stem, where they 
lie in a ring, as our pictures illustrate (figures 73, 139 B). The 
tissue in which they are embedded consists mostly of thin-walled 
cells, of rounded or polyhedral shapes. The part thereof lying 
inside of the ring of bundles makes up the pith, which is com- 
monly utilized for storage; that between the bundles constitutes 
the beginnings of structures later to be considered as the medul- 
lary rays; while the tissue outside of the bundles forms the cortex, 
which contains some chlorophyll, and aids in the photosynthctic 
work. This cortex, by the way, is continuous and morphologically 
identical with the green tissue of the leaf; and one can form a very 
useful and reasonably accurate conception of the anatomical 
relations of stem and leaf by imagining that one of the fibro- 
vascular bundles of the stem is snipped out from among its 
neighbors, and, with its adherent cortex, bent outward at right 
angles to the stem and then flattened and fringed out to a network 
which the green tissue surrounds and fills in. But as to our 
stem, outside of the tissues aforementioned comes the single layer 
of epidermis, physiologically the plant's skin, with its distinctive 
flat chlorophylless cells pierced here and there by the stomata. 
Finally, sometimes in connection with the sieve-tubes, some- 
times as a ring or as scattered islands in the cortex, or just under 
the epidermis, occur masses of very thick-walled cells, showing 
long and pointed when seen lengthwise, which are the important 
fibers that give strength to the stern. Howsoever these fibers are 
distributed, there is always one constant feature about their posi- 
tions, that they tend to keep close towards the outside of the 
stem. And the reason therefor is sufficiently plain, it is a 
fundamental principle of mechanics that any given amount of 
strengthening material exerts its greatest supporting effect against 
lateral strains if disposed in the form of a hollow cylinder or tube, 
which is the reason why columns used in building construction 
are hollow, not solid, why a bicycle frame is constructed of tubes, 
not of rods, and why a great tree can stand as a mere shell of 



How Substances are Transported and Removed 223 

wood long after its center has rotted away. It is true, this prin- 
ciple would require for greatest efficiency, that the fibers should 
lie on the very outside, as indeed they do in some cases; but such 
an arrangement would prevent all access of light and therefore 
the use of the surface for spreading of chlorophyll. It is easy to 
understand how the plant could find it advantageous to sacrifice 
a trifle of effectiveness in the strengthening system for the sake 
of the marked advantage of spreading more chlorophyll; and in 
this arrangement we see one of those innumerable compromises 
with which plants, like mankind, are accustomed to meet the con- 
flicting problems of existence. 

Such is the primary or ground structure of stems, as typically 
displayed in their earlier stages, and up to the time when they 
cease to be flexible, green and soft. Then they begin to undergo 
remarkable changes, connected adaptively with their continuous 
growth into trees; but these we can better postpone to our chapter 
on Growth, where the reader will find them fully described. 

It will interest the reader to know that the principal theme of 
this chapter, the transfer and transpiration of water, will al- 
ways be associated in the minds of plant physiologists with the 
foundation of their science; for to it, of all the phases of plant 
physiology, was first applied that exact scientific method of 
measurement which is the only sure means for advancing natural 
knowledge. Its founder was Stephen Hales, whose book Vegetable 
Statics, though published in 1727, might have been written' 
yesterday so far as its spirit is concerned. He will always be 
considered the father of this science, and his book one of the 
greatest of botanical classics. 



CHAPTER IX 

THE PECULIAR POWER POSSESSED BY PLANTS TO ADJUST 
THEIR INDIVIDUAL PARTS TO THE IMMEDIATE SUR- 
ROUNDINGS 

Irritability 




F the reader at this point will turn back to the Table 
displaying the plan of this book, he will see that we 
have now reached the end of our survey of the processes 
concerned with the nutrition of plants. These proc- 
esses are primarily internal, but they are all more or less depend- 
ent, especially for their supply of material or power, upon some 
one or the other of the external conditions. Now these external 
conditions, heat, light, water, minerals, and so forth, are never 
distributed quite uniformly around any individual plant, but are 
more or less abundant in some spots or directions than others. 
Obviously it would be a very great advantage to plants if each 
separate one of their parts, each leaf, stem, root, and so forth, 
could be adjusted or swung individually into the direction or 
position that would enable it to work to the very best advantage 
under the conditions presented by its own immediate surround- 
ings. Such a power, and in high degree of efficiency, plants in 
fact do possess, as we shall now proceed to consider. The reader 
will be surprised, I predict, by the importance and interest of the 
phenomena which belong under this head. 

We may best begin our study of the subject by considera- 
tion of its most familiar example. When a potted plant, like a 
" Geranium/' is grown in a greenhouse lighted evenly all around, 

it assumes a symmetrical form, alike on all sides, as everybody 

224 



Power to Adjust Parts to Surroundings 



225 



knows; but when the same plant is grown in the window of a room, 
where the light is wholly one-sided, it turns all its parts in that 
direction, even to the extent of seeming to reach out, as it were, 
after the light (figure 74). The same thing occurs commonly in 
nature, as may be noticed along the margin of shrubbery or close 




FIG. 74. Two "Geraniums" which for two or three days before their pictures were taken, 
were kept, respectively, in a uniformly lighted greenhouse and a chamber lighted only 
from the right hand side. 

to high buildings or banks; and it can be demonstrated very 
prettily by experiment (figure 75). 

A close observation of these cases shows always that stems and 
leaves behave very differently in relation to the direction of the 
light, for while stems point straight towards it, leaves set their 
faces across it. This suggests the inquiry, what, then of roots? 



226 



The Living Plant 



And for answer we turn to experiment. If seeds of mustard or 
radish are started in water-culture vessels, by methods described 
in an earlier chapter (page 136), the young seedlings grow rigidly 
upright in darkness; but if, when well started, they are given a 




FIG, 75. Sets of Radishes grown side by side in a chamber lighted wholly from the right 
hand side; but those on the instrument were kept continually revolving. 

one-sided light, they turn always as shown in our figure, the 
stems to the light and the leaves across it as before, but the roots 
distinctly away (figure 70). And such conduct is typical of or- 
dinary stems, leaves and roots. 

This process of light-turning is called in physiology Photo- 



Power to Adjust Parts to Surroundings 227 

tropism (pronounced with the accent on the second syllable), or 
Heliotropism. Parts that turn towards light are described as 
positively phototropic (with the accent, despite the seeming 
anomaly, on the third syllable), those that turn away as negatively 
phototropic, and those that turn 
across as transversely phototropic. 
Phototropism is so thoroughly typi- 
cal an example of the power of indi- 
vidual plant parts to adjust them- 
selves in relation to the immediate 
external conditions that we can use 
it as a basis for the analysis of the 
nature of this power, which is known 
physiologically, though not very 
happily, as Irritability. Now the 
elements entering into irritable re- 
sponses are these: 

First, the reason why the parts do 
it. As to this, the explanation 
must be amply obvious. The turn- 
ing towards the window brings the 
leaves into positions where they 
secure the best exposure to light, 
the light which is indispensable to the photosynthetic func- 
tion for which they exist. The best position for performance 
of this function must of course be that which sets them at 
right angles to the light; and this in turn requires that the 
stem, whose function is simply to carry the leaves, shall point 
or reach towards the light. As to the roots, not only does 
their function (the absorption of water and minerals), require 
no light, but their unprotected protoplasm is actually injured by 
exposure thereto; and this shows the advantage of their power to 
retreat from light. The reason for the characteristic phototropism 
of ordinary leaves, steins, and roots, respectively, is therefore to 




Fio. 76. A Mustard seedling germi- 
nated by water culture in darkness 
and then exposed to light falling 
from the direction of the arrow. 



^f-r 
j^ 



228 The Living Plant 

be found in an advantageous functional adjustment of those 
parts in relation to the direction of light. And this principle of 
advantageous individual adjustment of parts is characteristic of 
irritable adjustments in general. 

Second, the mechanical method whereby the turning is effected. 
The turning of the leaves, stems, and roots 
into their respective new positions requires 
both a considerable power and a definite 
mechanism. Now it is quite evident that in 
phototropism neither of these is supplied by 
the light, for that has no power at all to lay 
bdttily hold on the parts and forcibly pull, 
bend, or push them into their respective 
----- positions, while it is easy to prove on the 
contrary that the power is supplied by the 
- ^ plant, and derived from its own respiration. 

rThus, if oxygen be withdrawn from a cham- 
ber in which a symmetrical plant is sub- 
jected to one-sided light, not a trace of 
phototropic response ever follows. The con- 
nection will be clear to the reader: The 
Fro. 77.-Suecessive stages response requires energy, energy depends on 
in the downward turning respiration, respiration demands oxygen: 

of a root, showing, by the . / 

spread of the marks, that therefore no oxygen, no response. And as 

the apparent movement . ,1 \ / ,1 . - , i . i 

is effected i>y new differ- to the mechanism of the turning, that also 
aU is eSiS ^y determine(i b Y experiment, for if 



ready formed. The tn- stems, petioles, or roots are marked across 

angular piece is a paper . 

index. (Copied from with evenly-spaced lines before the plant is 
ectures.) exposed to one-sided light, then the marks 

spread apart in a way to prove that the bending accompanies 
growth in those parts, and is due to a more rapid growth on 
one side than the other, on just that side, indeed, where it is 
requisite in order to swing the parts concerned into the advan- 
tageous positions (figure 77). In phototropic adjustments, 



Power to Adjust Parts to Surroundings 229 

therefore, the already-existent tissues are not forcibly bent, 
but the new tissues grow in such an unequal or differential 
manner as to swing the parts into their new positions. In these 
respects phototropic responses are typical of others, for in all 
cases the power is supplied by the responding plant; and the 
motor mechanism consists, as a rule, in such differential growth, 
though occasionally it is of different sort, as we shall presently 
note. 

Third, the way the light operates in connection with the turning. 
Since it is not the light but the plant which accomplishes the 
turning, we still have to seek the nature of the role that light 
takes in the process. In brief, observation suggests and experi- 
ment proves that in phototropic responses the plant parts, which 
in general can grow quite as readily in one direction as another, 
use the light simply and solely as a convenient guide or signal 
(called scientifically, but not very fortunately, a stimulus), indica- 
tive of the most advantageous direction to take. It plays, indeed, 
very much the same part for the plant that the compass does for 
the sailor, establishing a definite line of direction, towards, 
across, or from which, according to circumstances, definite move- 
ments may be made. This case is typical of the action of stimuli 
in general; they never take any part in the mechanical accom- 
plishment of the irritable adjustments, but serve merely as signals 
for guiding, and sometimes for starting or stopping, the same. 

Fourth, the way the light stimulus is perceived by the plant. 
The plant has no eyes for the light, as the sailor has for his com- 
pass, yet it must possess some means of perception of the stimulus 
else obviously it could not react. The details of the matter are 
still much in doubt, but in general this much is certain, that the 
light falling on the sensitive protoplasm of the plant part sets up 
(probably by chemical means, since the blue rays are mainly 
concerned) a condition of irritation or strain, which puts the side 
towards the light in a condition different from the side away 
from it, and thus establishes the line of light direction. This case 



230 The Living Plant 

is typical of all stimuli, which act by producing in the sensitive 
protoplasm on which they impinge a condition of differential 
irritation or strain which serves to impress a line of direction on 
the part concerned. Then the part is swung by the motor mech- 
anism into a position where this condition of strain is the same 
all around, which position is kept in the subsequent growth. Ob- 
viously only those agencies can act as stimuli at all which can 
thus produce a differential state of the protoplasm, and con- 
versely, any agency capable of producing such a condition can, 
theoretically, act as a stimulus. And as to how strong a stimulus 
must be to produce an effect, it is only essential that it have 
enough power to produce the impression of differential strain 
on the sensitive protoplasm; and above that degree its strength 
does riot much matter. 

Fifth, how it is that a single uniformly-acting stimulus can evoke 
different directions of turning. The fact that in phototropism the 
light neither pushes nor pulls the parts to their positions, but acts 
simply as a guide to direction, involves the corollary which is 
confirmed by experience, that it is exactly as easy for parts of the 
plant to grow away from or across the light as towards it, pre- 
cisely as the sailor, guided by his compass, which neither pushes 
nor pulls him over the sea, can steer as easily to the south, east, 
or west as to the north where it points; and the reader should 
learn to think of all stimuli in this way. But if the parts of the 
plant can turn as easily in one direction as another in relation to 
light, what feature of their growth-mechanism is it which sends 
stems so unerringly towards it, leaves across it, and roots from 
it? Here again there is very great doubt as to particulars, but 
hardly any as to principle, which can thus be illustrated. In a 
locomotive, as most people understand, there is a certain lever, 
which when set in one direction determines that the engine shall 
move forward, and when set in another, that it shall move back- 
ward, after the steam is turned on; and an engine is easily imagi- 
nable in which, with the lever in yet a third position, the move- 



Power to Adjust Parts to Surroundings 231 

ment would be sideways. In all cases it is the same engine, the 
same machinery, the same motive power; the difference consists 
only in the way a small part of the machinery is set; and the 
reader will please to observe that this set of the machinery is not 
the cause of the movement of the engine, but merely determines 
the direction thereof when the power, which is steam, is applied. 
Now something of analogous kind, it is most probable, deter- 
mines the direction of turning of the plant organs. The structure 
and motive power in all of these parts is substantially the same, 
but in each some portion of the machinery is differently set, so 
that the application of the power, which is growth, causes turn- 
ing in the distinctive direction, the stem towards light, leaf 
across it, and root from it. Of course the machinery is not metallic 
but protoplasmic, and in last analysis is probably of a chemical 
nature, while, moreover, the set of the machinery is usually not 
alterable at a touch, but is hereditarily fixed in each kind of 
organ. And the subject may stand out yet more clearly if we 
return for a moment to our sailor, who, in order to reach a cer- 
tain eastern port, sets his steering gear to hold his good ship at one 
angle to his compass, and in order to reach a western port holds 
her at another. It is the same compass, ship, machinery, and 
power; only the set of the steering gear is different. This is the 
principle, I believe, which underlies the different kinds of responses 
to any single uniformly-acting stimulus. 

Sixth, how the advantageous direction of response has become fixed 
in each part. Or, in the simile of the preceding section, how did 
the machinery become set so differently in leaf, stem, and root; 
and especially, how did it become set in each of those organs in 
the manner most advantageous for the performance of its particu- 
lar function? Now it is perfectly plain that the power of a part 
to respond advantageously to a stimulus, that is to say, the set 
of its responding machinery, is an hereditary and adaptive 
feature, and must therefore have arisen in precisely the same 
manner as any other adaptive features, including those of visible 



232 The Living Plant 

structure, precisely, for example, as chlorophyll has been de- 
veloped in the leaf, a fibro-vascular cylinder in the stem, and 
hairs on the roots. Unless our whole philosophy of nature is 
wrong, there was a time when these things were not: now they 
are : at some time and in some way meantime they have arisen, and 
by gradual stages in the course of evolution. Our problem of the 
origin of the set of the machinery is therefore identical in kind 
with that of the origin of any adaptation, and thereby is trans- 
ferred into that separate field of inquiry which forms the subject 
of our later chapter on Evolution and Adaptation. 

The turning window-plant illustrates very clearly the nature 
of typical sensitive responses in plants; and all of the more com- 
plicated cases are identical in principle. Thus, not all stems 
turn towards .light, for those of wall-climbing Ivies (e. g. the 
Boston or Japanese Ivy) turn away from it, as manifest by the 
way in which these plants grow into porches and windows. The 
advantage, however, is evident on reflection; if these stems 
turned towards light, like the ordinary sort, they would be car- 
ried away from the wall and the possibility of clinging thereto; 
but, turning away from the light, they are flattened up against 
the wall where their holding discs can secure an attachment. This 
example shows also that no necessary connection exists between 
sternness, so to speak, and a set of the growth machinery towards 
light, but that the set is developed in the organs in correlation 
with their habits quite regardless of their morphological nature. 
Again, not all leaves set themselves across the light, for a good 
many kinds belonging in places very brilliantly lighted, like 
sub-tropical plains, set their edges to the direction of maximum 
brightness. In some this position is permanent, and may thus 
bring the leaves to a vertical north-and-south position, as in the 
Compass Plant of our prairies, which owes its name to this cir- 
cumstance; or, the leaves may change their positions, rising 
from horizontal to vertical at the time of maximum brightness, 
as in sundry plants of the Pea family (figure 78). The advantage 



Power to Adjust Parts to Surroundings 233 

of these vertical light-positions is believed to consist in a pro- 

tection given to the living substance of leaves against the full 

exposure to a brightness too intense for their good; for we know 

on the one hand, that too bright a light does chemical damage to 

protoplasm, even when partially screened 

by the chlorophyll, while on the other hand, 

leaves can make use of only a moderately 

strong light, the extra brightness being 

wasted upon them. It is this last-mentioned 

circumstance, by the way, which explains a 

problem that sooner or later will puzzle the 

reader, viz., why all the vegetation in the 

northern hemisphere does not have a turn 

towards the south where the sun is. This 

is no doubt because the diffused light falling 

on the plants from the north is quite as 

strong as they can use; and hence they have 

no object, so to speak, in turning to the side 

of the sun. 

There remains one other phase of photot- 
ropism in leaves which must here be consid- ,_ u A . .-,,.. 

^ 1 IG. 78. A plant of Mehlo- 

ered, and that is their lateral shif tings out tus > showing the position 

assumed in the bright 

from beneath one another s shade, a move- sun by the leaflets which 




ment chiefly accomplished by twisting and 
lengthening of the petioles. The result is paper by w. p. \\iison.) 
often to bring them, especially in spread-out plants like the 
vines, into a one-planed pattern where no leaf is overlapped 
by another, an arrangement commonly known as a leaf-mosaic 
(figure 79) ; and there are even some botanists who believe that 
the angular shapes of such leaves (e. g. in the English Ivy) are 
partly determined by the advantage of interlocking to use all 
the space. 

Such lateral shiftings imply that the whole upper surface of 
the leaf is equally receptive to the light stimulus; and a very 



234 The Living Plant 

ingenious and highly probable theory has been advanced in 
explanation, viz., that the epidermal cells, focussing the light in 
a special manner, are light-sensitive organs, and that the leaf 
keeps turning and shifting until all of these cells receive their 
full quota of light at the most desirable angle. In some other 
cases, however, the reception of the light stimulus is known to 
take place in a specialized spot, as for example in the seedlings 
of Grasses, which are light-sensitive only in the tip of the first 
sheathing leaf. The same thing is true, for several stimuli, of 
the growing-point of the root, and other cases are known. Evi- 
dently some such structures advance pretty far in the direction 
of the special sense organs of animals, such as eyes.* 

Thus much for the phbtotropism of stems, leaves, and roots: 
what now of flowers and fruits? As to flowers, they turn their 

* The localized reception of stimuli by the growing points of the roots is strikingly 
expressed by Darwin in the closing paragraph of his great book, The Power of Move- 
ment in Plants; and this passage illustrates so well a number of other phases of 
irritable responses that it is here reprinted in full. 

"We believe that there is no structure in plants more wonderful, as far as its 
"functions are concerned, than the tip of the radicle. If the tip be lightly pressed 
"or burnt or cut, it transmits an influence to the upper adjoining part, causing it 
"to bend away from the affected side; and, what is more surprising, the tip can 
"distinguish between a slightly harder and softer object, by which it is simultane- 
"ously pressed on opposite sides. If, however, the radicle is pressed by a similar 
"object a little above the tip, 'the pressed part does not transmit any influence to 
"the more distant parts, but bends abruptly towards the object. If the tip per- 
"ceives the air to be moister on one side than on the other, it, likewise transmits an 
"influence to the upper adjoining part, which bends towards the source of moisture. 
"When the tip is excited by light (though in the case of radicles this was ascertained 
"in only a single instance) the adjoining part bends from the light; but when excited 
"by gravitation the same part bends towards the center of gravity. In almost every 
"case we can clearly perceive the final purpose or advantage of the several move- 
" ments. Two, or perhaps more, of the exciting causes often act simultaneously on the 
"tip, and one conquers the other, no doubt in accordance with its importance for the 
"life of the plant. The course pursued by the radicle in penetrating the ground 
"must be determined by the tip; hence it has acquired such diverse kinds of sensi- 
"tivcness. It is hardly an exaggeration to say that the tip of the radicle thus en- 
"dowed, and having the power of directing the movements of the adjoining parts, 
"acts like the brain of one of the lower animals; the brain being seated within the 
"anterior end of the body, receiving impressions from the sense-organs, and directing 
"the several movements." 



Power to Adjust Parts to Surroundings 235 

faces, as a rule, directly to the light like the leaves, as anyone 
can observe in our house plants, or in those that happen to grow 
close to a building (e. g. a border of Nasturtiums), or against 
walls (e. g. Trumpet Creeper), or otherwise in one-sided light 
(figure 80). In a few flowers (e. g. Sunflowers), the phototropism 
even extends to the following of the sun through the day, though 
the adjustment is only moderately effective. Perhaps at first 
thought it will not be evident why flowers are phototropic at all, 




Fie. 79. The adjustment of Ivy leaves (of English Ivy) into one plane, affording the best 
aggregate exposure to light. (Copied, reduced, from Kerner's Pflanzenleben.) 

because, unlike the leaves, there is nothing in the function of the 
flower requiring the action of light. But on further contempla- 
tion of the use of the flower (a subject to be fully explained in 
the chapter upon Cross-pollination), and especially of the function 
of the showy corolla as an advertisement to show insects its 
position, the matter becomes evident; because obviously this 
function of conspicuousness requires that the corolla must stand 
out where the light can strike on it most fully. As to fruits, they 
are as a rule indifferent to light, though responsive to some 
other kinds of stimuli, as will later appear. One special case, 
however, deserves mention because illustrative of an additional 
fact about stimuli. There grows in Europe a little cliff-dwelling 
vine, Linaria Cymbalaria (figure 81), which turns its flowers as 
usual to the sun, but its ripening seed-capsules away therefrom. 



The Living Plant 



In consequence these seed capsules are brought into contact with 
the cliff, and, moving about more or less, are reasonably sure to 
push into some crevice where the seeds can be dropped in posi- 
tion for starting the new plant 
in its favorite habitat, instead 
of at the foot of the cliffs. 
There are two good reasons 
why I cite this example. In the 
first place it shows that the 
phototropism of a part may 
change the lever may be 
thrown during its own life, 
though this is not common. 
In the second place, the seed 
capsule has obviously no need 
to get away from the light as 
such, but simply to get back 
against the cliff. Since, how- 
ever, there exists no cliff-ward 
stimulus, the light, which hap- 
pens to act in the suitable di- 
rection, is used for the purpose. 
Light in this case acts as a 
foster-stimulus as it were, and 
may thus be described, in con- 
trast with the direct stimuli of 
the examples earlier described. 
There remain's one other class 

Fio. 80. A cut shoot of Bcllflower, kept for r r *. ,1 n T 

two days in a chamber lighted wholly Ot light responses, the SO-Called 

from the left. Observe the positive pho- s i eep movements. It is very 

totropism of the flowers. l J 

well known that some leaves 

droop at night, as in Clovers, Wood-sorrels, Beans, and many 
other members of the Pea family (figure 82); and most people 
have seen, at some time or other, the remarkably tight-shut ap- 




Power to Adjust Parts to Surroundings 



23? 



pearance presented by those plants at night. The same plants, 
moreover, can be put to sleep very easily, even at midday, by 
simply covering them up from the light. Now the exact meaning 
of the sleep movement is somewhat in doubt, as our chapter on 
Protection will show; but there is no question at all that light is 
the stimulus concerned. This response has, however, an interest 
in another direction, for the motor-mechanism is not growth, but 
a simple hydraulic contrivance contained in the clear little 
swellings at the bases of the sleeping leaflets. In the daytime, 




FIG. 81. The cliff-dwelling plant, Linaria Cymbalaria, showing the positive phototropisni 
of its flowers and the negative phototropism of its seed capsules, which thus are 
brought [into advantageous positions for the deposition of the seeds. (Copied, sim- 
plified, from Kcrncr's PJlanzenlcbcn.) 

under stimulus of light, their cells become strongly turgescent 
and hold the leaves stiffly expanded; but at night the turgescence 
is lessened, and the spring of the tissues, aided more or less by 
gravitation, causes them to droop. It is perhaps simply a high 
degree of development of sleep movement which gives us the 
remarkably-balanced leaf mechanism of the Sensitive Plant, 
later to be considered. 

In viewing these sensitive responses, and others of similar 
sort, one soon comes to wonder what the limits may be to the 
changes they can cause in the construction of the plant. This, 
like most of our problems, is amenable to experiment. If the 
most favorable possible conditions for one-sided stimulation are 



238 



The Living Plant 



supplied to a plant, it will turn to that side to a considerable 
degree; but the turning is never without limit, for, generally 
speaking, the farther it turns the more reluctant, so to speak, 
it is to turn any farther. If, on the other hand, the plant is so 
grown that it does not receive a one-sided stimulation, which is 




FIG. 82. A typical example of the sleep of plants. Both are Acacias, identical in kind arid 
age, but the one on the right has been covered for an hour from light. 

easiest accomplished by keeping the plant in continual rotation 
by aid of an instrument (called a clinostat) designed for the 
purpose (figure 83), then it always develops with remarkable 
symmetry, determined, very obviously, by internal and hereditary 
causes. The plant, accordingly, is born with an internal tendency 
to symmetrical form, but likewise with a considerable though not 
unlimited margin of possible deviation therefrom; and it is 
within this margin that the irritable responses take place. But 



Power to Adjust Parts to Surroundings 



2 39 



this margin has a greater interest than this, for it is characteristic 
of animals also, including ourselves, where it offers the basis for 
improvement of the body 
through exercise, and of the 
mind through education, 
while it is the field, as well, 
within which plays such free- 
dom as is possessed by the 
will. 

Phototropism has received 
this generous measure of 
attention because it is so 
thoroughly typical of irri- 
table responses in general. 
Accordingly the remaining 
forms of irritability can be 
treated much more briefly. 

Hydrotropism. If one pre- 
pares a porous clay germina- 
tor of the cylindrical form 
represented in our picture 
(figure 84) : fills it with water : 
hangs it horizontally : fastens 
small seeds along its sides: 
and places it in a chamber 
with a vapor-saturated at- 
mosphere, then the stems 

and the roots Will grOW Stiff- Fl0 :. *.-The clinostat, an instrument, whij-h 
" allows the effect of one-sided stimuli to be 

ly up and down as shown 

by the first of the figures. 

But if the surrounding air 

be partially dry, then the roots will cling close to the porous 

and water-soaked germinator, though the stems will act precisely 

as before. In the first case the moisture is the same all around; 




neutralized through the continual slow rota- 
tion of the plant. Note the resultant sym- 
metry of the Nasturtium whieh has been 
grown from seed on the instrument. 



240 



The Living Plant 



in the second it is most abundant on the side towards the ger- 
minator. The experiment, therefore, shows that roots turn in the 
direction where moisture is most plenty; that is, they possess a 
definite hydrotropism, another typical form of irritable response. 
The advantage of hydrotropism is perfectly evident when one 
recalls that the very first function of roots is the absorption of 




FIG. 84. Porous water-filled cylinders, to which seeds of Mustard were attached. That 
on the left was then kept in a saturated, and that on the right in a drier, atmosphere. 

water. The stimulus acts in this way; the water, absorbed 
more rapidly on the side of its greatest abundance, doubtless 
causes an osmotic swelling and tension stronger on that side 
than on the other; and this difference is ample to establish a line 
of direction towards which the roots turn in their growth. It is 
equally easy to see why stems and leaves display no hydrotro- 
pism at all, for, as they do not absorb any water under normal 



Power to Adjust Parts to Surroundings 241 

conditions, its one-sided abundance is a matter of indifference 
to them. This fact illustrates anew the adaptive character of 
these responses; for it is a general rule that plant parts are in- 
different to stimuli to which there is no profit in responding. 

The hydrotropism of roots involves matters of some practical 
consequence. It is said that when trees develop in a uniform 
soil, the root tips tend to collect in a circle just under the outer 
drip of the foliage, which is obviously the place where the water 
is usually most plenty. But in case the soil is moister on one 
side than another, the roots grow more freely in that direction, 
and may even extend to a distance several times the diameter of 
the tree. In their progress thus towards the most copious wet- 
ness, they sometimes are led to a drain, and, insinuating them- 
selves through some crevice left in the tiles, find therein a com- 
bination of water, air, and mineral substances so agreeable that 
they grow very profusely, even to so great a degree that they 
sometimes choke the drain quite completely. 

Chemotropism. But roots have also other irritable responses, 
notably to some chemical substances. Thus they turn, though 
rather feebly, towards a source of supply of some of the minerals 
they absorb, and this is typical chemotropism, with a very ob- 
vious advantage. But they turn much more strongly towards 
air (a special phase of chemotropism called AEROTROPISM), of 
course for the oxygen it contains, which they need for their 
respiration. It is easy to see in these cases how the stimulus is 
received by the root, for the chemical substance, especially the 
oxygen, must react with some of the materials found in the 
complicated protoplasm with which it first comes into contact, 
thus originating a differential chemical disturbance which would 
establish the line of direction. 

But other structures besides roots are markedly chemotropic. 
Thus pollen-tubes in their growth turn towards the substances 
secreted by stigmas and styles. In the fertilization of Ferns, an 
egg-cell at the bottom of a protective flask-like archegonium is 



242 The Living Plant 

fertilized by a male antherozoid which swims through the water 
(figure 104). Now when this egg-cell is ready for fertilization, a 
weak solution of malic acid pours out of the archegonium into the 
water, and diffuses steadily outwards. As soon as some wandering 
antherozoid perceives the presence of the acid, it turns and swims 
directly towards the source of supply, and hence to the egg-cell, 
which otherwise it would have no means to discover. And there 
is reason to think that such a secretion of special chemicals at the 
time when the egg-cells are ripe is very wide spread through the 
plant and animal kingdoms, providing the method whereby the 
swimming or growing male cells are enabled to find the female 
cells. This function is obviously not simply advantageous but 
indispensable. 

There are many important phases of chemotropism, but I 
have the space to mention only one more. Water-plants, which 
have floating leaves, alter the lengths of the petioles in accord- 
ance with the depth of the water, a matter which can be shown 
very beautifully by experiment. Now it is found that this regula- 
tion is chemotropic, or, more exactly, aerotropic, for, as ex- 
periment proves, petioles continue to grow until the leaves 
reach a supply of free oxygen, when they stop. This case illus- 
trates an additional fact about stimuli, viz. that they can serve 
as signals to stop a process as well as to guide it ; and other cases 
are known in which they act to start a process. Such stimuli 
are probably very important in controlling the various processes 
of growth, as our later chapter on that subject will demonstrate. 

Thigmotropism. This name is applied to those turnings and 
movements made in response to a touch as a stimulus. The 
most typical case is exhibited by tendrils, which, as the reader 
will recall, are those long slender structures sent reaching out for 
a support by a good many kinds of climbing plants. These 
tendrils sweep in long slow courses through the air until they 
touch some hard object, such as a stem, or a wire, around which 
they then curl in three or four turns (figure 85), thus obtaining 



Power to Adjust Parts to Surroundings 



243 






a grip which holds the vine firmly and permits a still farther 

ascent. Now it is easy to prove by experiment that it is really 

the contact with the support which constitutes the stimulus 

producing the bending, for anyone, by rubbing one side of a 

tendril with a pencil, can call out 

the turning, and watch all of the 

steps in its progress. Even a mo- 

mentary contact is followed by 

a turning within a few minutes, 

though the tendril will straighten 

again in case the contact is not 

maintained; but if the contact be 

continuous the tendril will wind 

completely around the pencil. The 

advantage, the motor-mechanism 

(which is growth), and the mode of 

reception of the stimulus, in this 

form of thigmotropism, are all suf- 

ficiently obvious. 

Most persons who have knowl- 
edge of plants would doubtless put 
forward a different case as a type 
of thigmotropism, viz., the well- 
known Sensitive Plant, which droops 
promptly and completely at a touch 

(figure 86). But I think this mOVe- F IG . 85. Four successive stages in 

mcnt is only accidentally thigmo- 

tropic. Nobody has yet found, 

. 

even after study of the plant in 
its native home, any satisfactory reason why the plant should 
droop for a touch, while, on the other hand, it responds in 
the same manner to other kinds of stimuli, a scorch of flame, 
a strongly-focussed light, a trifle of acid to which there can 
be no question of adjustment. The leaves have, however, 




Plified, from a wall-chart by Lau- 
rent and Errera.) 



244 



The Living Plant 



yet one other marked response, and that most important, be- 
cause, as is probable, it explains the original adaptation, 
viz. a marked sleep movement just like those which we have 
noticed already under phototropism. The motor-mechanism 
underlying the droop of the leaves of the Sensitive Plant is a 




Fit;. 80. Two Sensitive Plants, of which the one on the right was struck a sharp blow 
just before the photograph was taken. 

particularly efficient example of the hydraulic type already men- 
tioned; and probably it is so highly perfected and delicately- 
balanced that although developed originally in connection with 
sleep movements, it can now be set off, so to speak, by various 
other stimuli, such as touch, precisely, for example, as a cannon 
can be fired by a lighted match, an electric current, some chemi- 
cals, or a sharp blow. The sensitiveness of the Sensitive Plant to 
touch is upon this explanation accidental ; and there are probably 
yet other examples of such accidental stimulation in other phases 



Power to Adjust Parts to Surroundings 245 

of irritability. Indeed, in the very highly complicated and un- 
stable organization of the plant, it must often happen that the 
motor or growth mechanisms are set off, quite accidentally, 
by various wholly unrelated stimuli. Such is undoubtedly the 
nature of many of the " mechanical responses/' which by some 
recent writers have been made the basis of all plant activities, 
development and evolution, quite regardless of the innumerable 
other elements and conditions entering into the constitution of 
organisms. 

A good many additional cases of thigmotropic irritability are 
known. Thus, the leaves of some Insectivorous Plants close upon 





FIG. 87. Corn seedlings, showing the uniformity of position assumed by the growing roots 
and stems, respectively, from very diversely placed seeds. 

flies that alight upon them, quickly in the Venus Fly-trap, and 
slowly in Sundew. Some stamens when touched by insects, move 
up in such a way as to dust those visitors thoroughly with pollen, 
thus aiding in the utilization of insects for cross-pollination of 
flowers, of which the importance will later become apparent to 
the reader. In these and some analogous cases, the advantage, 
mechanism, and method of stimulation are all more or less well 
understood. 

Geotropism. When seeds fall to earth, or are placed in the 
ground by a gardener, they come to rest in the most diverse 
positions, with their embryonic roots and stems pointing at any 
and all angles. Nevertheless, as they germinate, the young 
roots, with a singular unanimity, turn downwards and the stems 
upwards. The same thing can be shown very clearly by ex- 



246 



The Living Plant 




periment, for if a number of large seeds, such as Windsor Beans, 
or Corn, be fixed in the most diverse possible positions (figure 87), 
the new stems and roots will grow themselves round into the 
up-and-down directions respectively. Furthermore, the side 

roots as they come out, and side 

*v J^V branches as well, assume and hold 

"^1 S for a time a definite angle to the 

same up-and-down line. That the 
positions of these parts are taken 
with reference to the up-and-down 
line, and not simply in relation 
to the main root and stem, is 
proven by a very conclusive ex- 
periment; for if the young plants, 
when their parts are well formed, 
are tipped over at an angle, or up- 

Fw. 88. This Bean seedling was grown side down as shown by Olir figure 
for a time in this position; then it was /n oo\ u 11 r J.T~ 

inverted, and the new growth is reprc- (figure 88), then all OI the parts 

^S'S^ grow as quickly as they can into 

still further growth. The direction of their former directions. A case 

growth is obviously geo tropic, not 

relative to the main root. (Copied, of analogous SOrt is found also 

reduced, from Sachs' Lectures.) ^ T , , , 

in Nature, where evergreen trees 

that grow on irregular steep hillsides show no relation what- 
ever to the slope of the ground, but grow as stiffly upright, 
and with branches as truly horizontal, as if the ground were 
quite level. These simple illustrations are typical of a well- 
nigh universal fact about plants, that they send their first 
roots down and their first stems up, and their side roots and 
side stems out at definite angles to the up-and-down direc- 
tion, regardless of the conditions under which they originate. 
This fact is fundamental in the economy of vegetation, for it 
helps to explain the way in which large plants can guide their 
growth into upright positions, and hold themselves therein, 
and how they can spread out their branches at such definite 



Power to Adjust Parts to Surroundings 247 

angles as to give to these plants their characteristic outlines. 
Furthermore it also explains how stems can so readily recover 
their natural positions when the plants are over-turned, whether 
by accident, or by intention in experiment. 

We must next turn attention to this crucial matter of the 
up-and-down line. Now there is in this world only a single 
determinant thereof, and that is the attraction of gravitation, 
which forever is drawing all objects towards the center of the 
earth. Gravitation, therefore, would seem to be the stimulus 
used by the plant in assuming the positions we are considering. 
In other words, the parts of the plant are gcotropic ; and all evi- 
dence confirms this conclusion. 

The wide use of gravitation as a stimulus raises at once the 
question as to the physiological value of gravitation to the plant. 
In itself, however, it has no value, so far as anyone has been able 
to discover. The plant has no object at all in sending roots 
downward and shoots upward merely to have them down and 
up; but it happens that down is the direction of moisture and 
minerals, which roots need, and up is the direction of light, which 
shoots need. No doubt those parts could be guided in the need- 
ful directions by their hydrotropism and phototropism respect- 
ively, but gravitation has this advantage over moisture and 
light as a stimulus, that, while happening to act in the suitable 
direction, it is present unvaryingly at all times, whereas light 
and moisture are most variable in quantity, and sometimes 
absent altogether. This is especially true of light, which is missing 
at night when growth is most active and the guiding stimulus 
most needed. Gravitation, therefore, is neither a direct, nor a 
foster stimulus, like those we have already considered, but a 
substitute stimulus, adopted by the plant in place of other 
stimuli because it acts better than they. The use of the compass 
has just the same advantage over observation of the sun and the 
stars, which would also take the sailor to his port; for the compass 
is constant in its action, while the sun and the stars not only 



248 The Living Plant 

vary in direction all through the twenty-four hours, but often- 
times are obscured altogether. Moreover, this principle of sub- 
stitution stimuli is often important in connection with the de- 
velopment of structures, for it helps to explain how an organ or 
other feature can form in advance of perception of the stimulus 
to which it is later to react, e. g. the formation of the eye before 
birth in animals, and of chlorophyll in the embryos of plants. 

The way in which the gravitation stimulus is perceived by the 
plant seems clear. Gravitation draws the heavier contents of 
the cells, especially the starch grains, down to the bottom of the 
cell, where their weight presses hard on the sensitive protoplasm 
and produces a condition of strain different from anything in the 
upper part of the cell; and this difference establishes the line of 
direction. Then the responding mechanism is so set that main 
roots are sent growing towards this pressure, main stems away 
from it, and side parts across it, precisely as in other typical 
responses. Geotropism, by the way, is a perfect illustration of 
the fact that a stimulus acts merely as a guide, and not as a 
physical aid, to responses; for while gravitation might be sup- 
posed to help pull roots downward, obviously it cannot be 
imagined to help push stems upward or to drive side parts out 
crossways. 

Thus much for the geotropism of stems and roots; what of 
leaves, flowers and fruits? As to leaves, their geotropism is 
usually disguised by their stronger phototropism; but that they are 
geotropic is shown by the vertical or horizontal positions they 
assume when kept in dark rooms. We see another illustration 
thereof, as I take it, in Nature, in some of the broad-leaved 
shrubs which grow in the shade of the forest; here the diffused 
light is so evenly distributed that it exerts no one-sided stimulus, 
and the leaves are left free to assume their geotropic position, 
which is strikingly horizontal. As to flowers, they also, for the 
most part, are definitely geotropic. Thus, if one selects a long 
terminal cluster of unopened irregular flowers, such as Larkspur 



Power to Adjust Parts to Surroundings 



249 



or Snapdragon, bends it over and fastens it down at the tip, as 
shown by our figure (figure 89), then each of the blossoms, as it 
opens, turns over individually to the very same position it would 




FIG. 89. Flower shoots of Larkspur, the curved one of which was bent over and fastened 
a few days before this picture was taken. Note the uniform geotropic positions as- 
sumed by both buds and flowers. 

have had in the vertical cluster. The position of each separate 
flower is here established geotropically, and for a very good rea- 
son, viz., these irregular flowers, as our later chapter on the 



250 The Living Plant 

subject will show, are specialized for cross-pollination in a way 
which makes the lowermost petals alighting places for insects; 
and therefore these petals must be kept horizontal. For the 
same reason the long tubes of Daffodils are geotropically hori- 
zontal, as one can prove by fastening the young flower-stems in 
horizontal positions; and there are other cases without number. 
As to fruits, they are mostly indifferent geotropically, but a few, 
e. g. Cyclamens and Pea-nuts, use gravitation as a guide as they 
bury their seeds in the earth. 

So many and interesting are the manifestations of geotropism 
in special cases that I must take room for a few more examples. 
Trailing vines, whose main stems rest flat on the ground, like the 
Periwinkle, Twin-flower, and Ground Pine, and perennials with 
horizontal stems just beneath it, like Solomon's Seal, keep these 
positions by virtue of the fact that their main steins have not the 
usual main-stem geotropism, which is upright, but the trans- 
verse kind characteristic of side-branches; twining plants are 
kept encircling a vertical support under guidance of a lateral 
geotropism, and this is what prevents them from twining around 
horizontal branches or supports which would not take them up 
towards the light; the aerial roots of many tropical climbers, and 
most tendrils, have likewise this lateral geotropism, which keeps 
them swinging horizontally until they meet with a support; and 
there are many other cases of which some may be identified by 
the reader himself if he keeps observationally alert in his walks 
abroad in field, garden, or forest. 

Of all of the stimuli made use of by plants for guiding their 
parts to positions of greatest advantage, gravitation is much the 
most important. Plants are born with an hereditary tendency 
to put forth their parts in a symmetrical manner, as can be 
demonstrated experimentally by aid of the clinostat; but they 
depend upon geotropism to guide those parts into the suitable 
positions, and thus to realize the ultimate shape of the plant. 
And this is the case no matter what the form of the plant may be, 



Power to Adjust Parts to Surroundings 251 

whether a symmetrical cone of horizontally-spread branches ra- 
diating from a central main stem, as in the Firs or the Spruces, 
or a great urn of up-and-outcurved branches, as in the Maples 
and Elms, or in any of the intermediate shapes; and the reader 
should learn to visualize all of the main trunks and branches as 
thus developing in touch with gravitation and largely under its 
guidance. This applies, however, only to the main structures; 
the smaller branches and most of the minor parts are more or 
less controlled by other kinds of stimuli which determine the 
final details of form; and this is especially the case with roots. 
The fact that geotropism is thus ever tending to hold the plant 
to a certain upright symmetrical form explains why any one- 
sided turning in response to other stimuli, is of limited amount, 
and why the plant always tends to recover its former upright and 
symmetrical position in case it is disturbed. 

Some minor tropisms. These include, THERMOTROPISM, a turn- 
ing towards warmth, rather rare: TRAUMATROPISM, the turning 
of roots away from an external irritation or injury: RHEOTROPISM, 
a turning against a water current, which, however, has been 
shown to be only a special phase of thigmotropism : ELECTRO- 
TROPISM, a certain adjustment to mild electric currents; and some 
others of lesser importance. The case of rhcotropisrn, by the 
way, illustrates a confusion of stimuli, the root apparently mistak- 
ing the pressure of the flowing water for that of some hard ob- 
ject in the soil. The case of electrotropism, involving response 
to an influence to which the plant is never subjected in nature 
and to which it cannot have become adaptively sensitive, illus- 
trates the same thing, or else, perhaps, an accidental release of the 
motor-mechanism after the manner already described for the 
Sensitive Plant. And the occasional responses found in plants 
to other stimuli new to them (e. g. to X-rays, radium emana- 
tions), are likewise due without doubt to confusion of stimuli, or 
accidental release. 

Thus far we have considered for the most part only cases in 



252 The Living Plant 

which the stimuli act from a single direction, and therefore evoke 
only one-sided responses. But some of the very same stimuli 
may act in a diffused or all-around manner, becoming impressed 
on the sensitive protoplasm of the plant through a change in 
intensity; and in such cases the responses are all-sided or sym- 
metrical. Thus the sleep movements of leaves, already con- 
sidered, are of this nature, being a response to the change in in- 
tensity of the circumambient light; and the same thing occurs 
with some flowers, which close at night or in very dark weather. 
Other flowers, e. g Tulips, are affected in like manner by changes 
of temperature, opening as the weather grows warmer, and clos- 
ing as it becomes cooler; and some evergreen leaves, notably of 
Rhododendrons, rise and fall in this way even in winter. Such 
responses are distinguished from the ordinary sort in scientific 
terminology by the termination, nasty (photonasty, thermonasty, 
etc.); and we may note by the way, that the responses due to a 
free-swimming movement, as in the case of the antherozoids of 
Ferns already described, are distinguished by the termination 
taxis (chemotaxis, phototaxis, etc.). 

There are, furthermore, several other types of responses to 
stimuli, some of them vastly important in connection with the 
growth and development of plants. Thus, it has been claimed 
that the strains set up by the swaying of stems back and forth, 
whether in nature by winds, or in the laboratory under experi- 
ment, serve as stimuli to the larger development of strengthen- 
ing tissues in the places where the strains are most felt, thus pro- 
ducing a needed enlargement at those places. It is perfectly 
clear that the great knees which rise from the roots of the Bald 
Cypress of the Southern Swamps and which probably are aerating 
structures, are formed in response to the presence of water, for 
they do not form at all when these trees grow in soil that is well- 
drained. Other cases are known where the thickness of cell-walls, 
the arrangement of tissues, the sizes of parts, and other structural 
features are regulated by responses to well-known stimuli from 



Power to Adjust Parts to Surroundings 253 

the environment. Again, the climbing roots of some Ivies, and 
the sucking roots of some parasites, grow out at those places 
where the stimulus of contact is felt, and therefore exactly at the 
places where they can best serve their uses; and the places of 
origin of even ordinary roots are largely controlled by the stimulus 
of especially abundant moisture or minerals, which explains why 
roots branch so profusely upon entering drains. Then there are 
stimuli which start particular stages of growth. Thus it is a 
stimulus given by some phase of fertilization which starts the 
formation of the fruit in the higher plants. The advantage is 
clear, since the fruit would be wasted, and its formation a useless 
drain on the plant, if no fertile seed were produced; for the dis- 
persal of the seed is the function for which the fruit exists. Stimuli 
can also serve as signals to produce a cessation of growth, as in 
the case of the leaves of the water-plants already considered; 
and there are plenty of other cases where stimuli regulate growth 
and development in various ways, the further consideration of 
which we may postpone to the chapters which deal with those 
subjects, where also we may consider the correlation and linking 
of stimuli, with their very important consequences. 

There is one other phase of responsiveness to stimuli which we 
must consider at this place. It is a familiar fact about organisms 
that they have a certain power of adjusting themselves, or be- 
coming toned, as it were, so as to work their best under the pre- 
vailing conditions to which they are exposed; and when they are 
thus working in full harmony with those conditions they are said 
to be in tone. We have a familiar illustration thereof in our 
human affairs in the way we become accustomed to certain pe- 
culiarities of food, temperature, fresh air, occupations, etc., to 
such a degree that we become uneasy when exposed to any others, 
and hasten back with relief to the congenial conditions. Thus, 
most of us work our best at about 70 Fahrenheit and become 
very uncomfortable when the temperature rises to above 90, 
though this is still much less than the natural heat of our bodies. 



254 The Living Plant 

Moreover this condition of tone is more or less alterable under 
continuous action of new conditions, and such tonic adjustment 
to new conditions is commonly called acclimatization. We do not 
yet know much as to the nature of the process, but there seems 
little doubt that it is chemical in its nature, and represents a 
process of chemical adjustment to the external conditions acting 
as stimuli. An important phase of the same process is found in 
the formation in the animal body of those special chemical sub- 
stances called collectively "antibodies," which neutralize chemi- 
cally the injurious substances formed in disease. Probably the 
acquisition of tone and acclimatization are fundamentally similar 
in principle, consisting in chemical alterations in the protoplasm 
of such character that substances or features less efficient under 
the prevailing conditions are replaced by others more efficient. 
At least such seems to be the principle, though as to the details, 
they are still with the future. 

As one views the various adjustive structures produced in 
response to external stimuli (such as the knees of the Bald Cypress 
just mentioned, the thicker epidermis of plants in dry places, 
and so forth), one cannot but ask how these may be distinguished 
from adaptive structures produced in the course of evolution; 
and whether, after all, the two may not be fundamentally the 
same thing. As to the first point, one cannot distinguish adjustive 
from adaptive structures by any evidence except the test of 
heredity, for adjustive structures are produced anew in each 
generation only in response to certain stimuli and are absent 
when the stimuli are lacking, while adaptive structures are pro- 
duced regularly every generation quite regardless of the presence 
or absence of the given stimulus. The only thing that is hereditary 
in irritable adjustments is the capacity to make them. We have 
an analogy in the different methods whereby republics and 
monarchies are provided with rulers, for while the president of a 
republic is often indistinguishable in mode of life and other 
characteristics from a monarch, and may even surpass one in 



Power to Adjust Parts to Surroundings 255 

power, he is chosen quite anew at regular intervals in adjustment 
to the popular demands of the moment, only the method of elect- 
ing him being permanent, or, so to speak hereditary; while the 
monarch holds his office by heredity quite regardless of the 
fluctuations of politics. As to our second question, whether in 
the last analysis, the two may not be fundamentally the same, 
adaptive structures being only permanently-fixed irritable ad- 
justments, the view is attractive but as yet unproven, as we shall 
further consider in the chapter on Evolution. 

There remains one other important matter to mention in con- 
nection with stimuli. The response to a stimulus, while highly 
efficient, is blindly invariable, and not alterable for particular 
conditions. For example, if a wind-blown seed of an ordinary 
plant were to lodge in a cleft of an overhanging ledge, it would 
be an advantage for this plant to be able to reverse the usual 
positions of roots and stem; yet we know it would send its stem 
up, though only to die in the earth, and its root down, only to 
perish in the air. In this invariability of particular responses, 
and in many respects besides, these irritable responses of plants 
agree with the reflex actions familiar in animals; and it is now 
very clear that they are essentially the same. Furthermore, if 
two or more stimuli act upon the same part of the plant at the 
same time, the result is simply the product of the effort of the 
part to respond to them all. There is no sign of an attempt on 
the side of the plant to correlate these stimuli, so to speak, and 
to respond in a manner which would be best in the face of this 
particular combination. In this respect animals have gone far 
ahead of plants, for they have acquired that last-mentioned 
power. Herein we have the chief feature which distinguishes 
the higher animals from the higher plants, and also, I believe, 
the origin of consciousness. Thus, out of one and the same 
origin, plants have developed irritability, while animals have 
developed reflex action, consciousness, and ultimately reason. 



CHAPTER X 

THE VARIOUS WAYS IN WHICH PLANTS RESIST THE 
HOSTILE FORCES AROUND THEM 

Protection 




]Y the methods considered in the preceding chapters, 
plants provide most effectively for their nutritive 
needs, and also for advantageous adjustment to the 
external conditions affecting the same. But they have 
not thereby solved the whole problem of daily existence, for they 
still have to reckon with the presence of a great many hostile 
external conditions. Thus the winds, which in moderation do 
no damage to plants and even may work them some benefit, 
occasionally swell to great tempests possessing a power well- 
nigh too great for resistance. Again, water, which is indispensa- 
ble to plants in considerable quantity, becomes sometimes, 
through drought, quite dangerously scant, or through floods 
quite as dangerously plenty; while various parts and places of 
the earth, deserts on the one hand and swamps on the other 
though perfectly habitable by plants in all other respects, remain 
permanently in one or the other of these undesirable conditions. 
Further, light, which is likewise essential to plants, is in some 
times and places too weak for efficiency, and in others so intense 
that unprotected protoplasm can by no means endure it. And 
again, the food supply manufactured by plants, while ordinarily 
ample for both themselves and their hereditary dependents the 
animals, is in some parts indispensable to the continuance of the 
plants' activity, so that its destruction by animals would consti- 
tute a serious menace. Finally, while endowed with indefinitely 

256 



How Plants Resist Hostile Forces Around Them 257 

great powers of reproduction and growth, plants live in a world 
already quite filled, and are therefore exposed to a competitive 
struggle with one another, of which natural selection is the re- 
morseless arbiter, and a survival of the fittest the inevitable 
outcome. In a word, plants live in a world that is generally 
friendly, but sometimes is hostile even to a mortal degree. 
Against the hostile features of the environment they have had 
to develop protective adaptations, some of which are extremely 
conspicuous and play a large part in the determination of the 
habits and aspects of plants. These protective adaptations, of 
course, must co-exist and compromise with those physiological 
adaptations in leaf, stem, and root, which we have already con- 
sidered. The identification, separation, and definition of the 
structures and features of plants which are protective is the task 
that now lies before us. 

To begin with, the protoplasm of plants is physically weak, 
but secures an efficient first line of defense by the most obvious 
of all methods, viz., through encasing each one of the soft-bodied 
cells in a separate coating of armor, the cell-wall. As the 
reader will recall, from the description in the chapter on Proto- 
plasm, the plant skeleton is constructed from the united wall- 
mass of the cells; and it thus combines both support and seclu- 
sion for the protoplasm in its cavities, very much as the walls of 
our many-storied houses do for us. Such a combination of skele- 
ton and protecting wall is permitted only by the sedentary habits 
of plants, and stands in very great contrast with animals, whose 
locomotive habit requires a jointed skeleton, moved by masses 
of contractile, and therefore naked (muscular) cells. 

Turning now in detail to the various hostile influences against 
which plants need protective adaptations, the most obvious is 
that of the winds, which, however, become a danger only as they 
rise into gales. Then, as all will agree who have seen a great 
tree tossed in the grasp of a tempest, protection is found in the 
slenderness and elasticity of the branches, which yield in great 



258 The Living Plant 

curves that permit the smaller to stream with the wind in the 
lee of the larger, where they can tug at their anchorage in safety. 
Doubtless in a windless world the plant skeleton would be rigid 
and brittle, probably to such a degree that an ordinary one of 
our storms would shatter it to fragments, much as at times they 
do now with the ice of a silver thaw. As to older steins, we have 
learned already how it is with them; their hollow-column princi- 
ple of construction holds them up against great lateral strains. 
Furthermore, a good many kinds of stems exhibit a special 
strengthening arrangement at the place of maximum weakness, 
which lies at the contact of stem and root, where the leverage 
exerted by wind on the top is most felt. Thus, some kinds of 
plants, like the Corn, develop prop roots that extend from the stem 
above ground diagonally down to the earth, while many tall 
trees possess buttress-like thickenings between the stem and the 
principal roots, as appears very well in some of our Elm trees, 
and especially in some of the tropical giants, where they attain a 
good many feet of height and breadth, though only a few inches 
of thickness. As to leaves, whose broad faces would present 
much exposure to wind, their slender-elastic petioles permit them 
to yield, and to swing like so many weather vanes, presenting 
only edges to the blast, while they can also sway accommoda- 
tingly to every irregular gust. In this adaptation, indeed, we 
find one of the principal functions of the petiole, as follows from 
a discovery made by one of my own students, who found that 
the petioles from the exposed part of a tree average longer than 
those from more sheltered situations, although the leaves are 
smaller in the former locations than the latter. 

But it is not alone on the individual tree that the sizes of leaves 
are inversely proportional to the degree of their exposure tc 
winds, for it is true in general of plants as a whole. Do not the 
largest leaves that are known to the reader grow in the sheltei 
of undergrowth? And if at first sight it appears that the gigantic 
fronds of Palms and Tree Ferns contradict this view, a seconc 



How Plants Resist Hostile Forces Around Them 259 

thought is enough to confirm it; for, although morphologic- 
ally single leaves, they are cleft to a great many small leaflets, 
each of which acts physiologically as a single leaf. This division, 
or " compounding " (as it is. called scientifically), of leaves in such 
plants appears clearly to constitute a protective adaptation 
against the tearing action of winds; and I believe the same factor 
is the principal one in determining the compounding of leaves in 
general, though sometimes the compound condition, as in our 
undergrowth Ferns, means rather a persistence of an ancestral 
condition than anything of immediate importance. Nor is one's 
natural thought at this point, that the sizes of leaves are de- 
pendent on their thickness, correct. The thickness of leaves is 
determined by the depth to which sunlight can penetrate green 
tissues without losing all of its photosynthetic power; and hence 
it is approximately the same in all leaves exposed to the sun in 
the same climate, with a trend towards more thickness in extra- 
bright places, and thinness in shade. Undoubtedly the whole 
tendency of wind action is to produce an adaptive lessening in 
size, which is directly antagonistic to the tendency of photo- 
synthesis to produce a larger spread of surface; and the resultant 
between the action of these two factors, modified it is true by 
certain other minor influences, makes leaves the sizes they are. 
This explains why our common deciduous trees of similar habit, 
our Oaks, Elms, Maples, and Chestnuts, possess leaves of much 
the same size, or at least of the same order of magnitude. That 
size represents the equilibrium between the contesting photo- 
synthetic and wind factors acting on leaves of standard thick- 
ness growing in similar situations. 

Another kind of strain to which plants are exposed is the 
weight of the winter's snow and ice. This danger is greater, of 
course, for evergreen than deciduous trees, but against it the 
conical shape characteristic of evergreens provides a manifest 
protection. This follows from the fact that only the ends of the 
branches are exposed to the falling load, while their slender forms 



260 The Living Plant 

and horizontal positions permit them to yield greatly without 
damage, and thereby even to shed their burdens (figures 14, 15). 
No doubt the protective adaptation involved in the conical shape 
has operated along with the photosynthetic considerations 
earlier mentioned (page 56) to fix this form for evergreen trees, 
which in general are commonest in the snowiest regions; while, 
correlatively, the danger involved in the accumulation of snow 
upon the leaves borne by upwardly springing branches, like those 
of most of our deciduous trees, is doubtless one factor in making 
such trees drop their leaves in the winter. 

This mention of the shapes of trees makes this a suitable place 
to consider their modes of resistance to certain other strains. 
The stems of trees have not only to carry great masses of foliage 
high up in the air, but also to support it out laterally for con- 
siderable distances, and all in opposition to a heavy downward 
strain from gravitation. In some trees, conspicuously those of 
the cone-shaped evergreen type (figures 14, 15), the branches 
spread horizontally from a central upright trunk; but this ar- 
rangement, however advantageous from other points of view, 
is mechanically the worst for resistance to gravitational strains, 
and is only possible with comparatively slender branches and 
special methods of strengthening the same. Thus, bracket-like 
swellings often occur in the angles between such branches and 
stems, while extra material is commonly placed all along the 
under side of the branch, making it excentric in cross section. In 
such cases the extra material acts much like a long stiff spring 
bent upward just enough to counterbalance the weight of the 
branch, whose horizontal position is maintained by the counter- 
action of the two forces, as is shown quite conclusively by the 
very great bending of such branches when spring and weight are 
allowed to act together by the inversion of the tree (figure 90). 
But a cone-shape of trees is uncommon in comparison with that 
in which great branches, often well-nigh as large as the trunk, rise 
up therefrom at sharp angles, swing gradually outward to near 



How Plants Resist Hostile Forces Around Them 261 




Tracings from photographs of the same 
Balsam Fir, in tho natural position and inverted, 
illustrating a point explained in the text. 



the young parts, and then curve vertically upwards again to 
bear the new leaves, the whole stem melting away, as it were, 
to a spray of such branches. This is the form prevailing in most 
of our deciduous trees, as the reader can see for himself by 
examining the tracery of 

Oaks, Maples, Elms, or ^x^ V 

Chestnuts when projected 
against the winter sky. 
Such a sigmoid form of 
the branches affords them 
the best possible anchor- 
age in the trunk with the Fl( , (M) 
minimum of leverage on 
their heaviest parts, while 
providing enough spread and a vertical tip for support of the 
foliage. If, now, we apply this sigmoid mechanical modification 
to the theoretical form of our photosynthetic tree represented in 
figure 7, we obtain the form illustrated herewith (figure 91). 
This theoretical form, modified by some minor and largely acci- 
dental circumstances, is very nearly realized in the noble Oak 
shown in figure 8, and by many of our common deciduous trees. 
The chief difference consists only in this, that whereas the 
theoretical tree is hemispherical, the actual kinds are often 
ovoid, cylindrical, or top-shaped, in obvious adaptation, as I 
think, to a diminution of the excessive gravitational leverage 
that accompanies too extensive a spread. 

We pass now to a second of the greater environmental in- 
fluences hostile to plants, namely excessive light. The reader 
does not need to be told that light, and in large quantity, is in- 
dispensable to plants for their photosynthetic work; but it is an 
important physical fact that the amount they can thus use has a 
limit, above which any increase is not only useless but positively 
harmful. And that limit is often surpassed in the open sunlight 
of summer. However, not all of the mani-colored rays that make 




262 The Living Plant 

up the white light are thus injurious, but only the blue-violet, 
and then only when received in great force; for these very same 
rays, like some of the red,, are the ones that are useful in photo- 
synthesis. They produce their bad effects, as it seems, through 

their peculiar power of promoting 
chemical changes, whereby they in- 
duce in the complicated living 
protoplasm illegitimate reactions, 
as it were, which interrupt the or- 
derly series of chemical processes 
in which the very life of the 
protoplasm consists. However, 
whether this be the correct ex- 

Fi. HI. The theoretical form of a de- , . . , , 

ciciucms tree, consisting f the photo- planatioii or not, it is nevertheless 

synthetic groundwork shown by fig- n f np | fl,,^ f rrmo , im pTv>miml lio-Vil 
lire 7 modified in adaptation to the d ItlCt tn<it stron S Unscreened light, 

mechanical support of the weight of because of its blue rays, is always 

the foliage. ... 

injurious to living protoplasm. This 

is the reason why bright light is fatal to disease germs, or Bacteria, 
and explains the basis of the hygienic value of sunlight in the 
home; while blue light is used with success for the very same reason 
in the cure of some diseases of the skin. Now because the red 
rays of the sunlight are not only harmless but also useful, even in 
fullest intensity, while the blue rays are harmful only when in- 
tense, but otherwise useful, the problem of adaptive protection 
against too intense light resolves itself into one of tempering the 
blue rays without affecting the others. This can be perfectly 
accomplished through use of a screen which permits red rays to 
pass while checking the blue, and such a screen is of necessity 
red. It is upon precisely this principle that photographers use a 
ruby glass screen in developing their plates, for this color cuts 
off the blue rays, which are those that took the picture originally 
and therefore would spoil it in development, while admitting the 
red rays which arc not only harmless to the plate but useful in 
showing the photographer what he is doing; only the photog- 



How Plants Resist Hostile Forces Around Them 263 

rapher needs a total exclusion of blue rays and therefore a screen 
of much deeper color than the plant requires for only a partial 
exclusion of those rays. Such is most likely the adaptive signifi- 
cance of that charming red blush which mantles the face of the 
fresh vegetation of spring, for, without some such protection, 
the young leaves and stems that push out of the buds before 
the formation of the chlorophyll, which constitutes later a suffi- 
cient though incidental protection, would expose their unshielded 
protoplasm to the full force of the bright light then prevailing. 
And there are some students who find a similar function in the 
redness of leaves in the autumn, believing that it shields the 
protoplasm after the chlorophyll has faded away; though here, as 
I believe and have argued in the second chapter, there is little 
warrant in the evidence. Certain it is that there are cases, e. g., 
the red under sides of leaves of some tropical undergrowth 
plants, where the explanation must be totally different. But 
the light-screen function explains very well the reddish or brown- 
ish colors of spores which must float long-time in the air exposed 
to the brightest of light, and perhaps it explains also the red 
color assumed by roots and underground stems when these be- 
come exposed to the light, though here the color may represent 
simply a chemical incident. 

A second method of light protection may consist in those hairy 
or woolly coatings, or even in the waxy or resinous layers, which 
overspread a good many plants of open bright places, resulting 
in a distinctive aspect of grayness found especially often in plants 
of the deserts. Such covers must act to reflect and refract the 
light, without, of course, any distinction of rays, to an extent suf- 
ficient to weaken very greatly its power to penetrate the tissues. 

The third of the methods of light protection, bound up, how- 
ever, with protection against excessive transpiration soon to be 
noted, is more important. It consists in the assumption by the 
green tissues of a vertical position, whereby they present only a 
thin edge, or at least a low angle of incidence, to the mid-day 



264 The Living Plant 

brightness of the sun, with the full exposure to its less intense 
action at morning and evening. Such a vertical position of the 
green surface is common in plants of open bright places, in 
some, notably certain clover-like kinds, as a temporary and 
irritably-adjustable position of the leaflets (figure 78), but in 
others as a permanently vertical arrangement of the leaves. In 
the most perfect of the latter cases, all the leaf-blades present 
their faces to the east and the west, thus bringing their edges 
north and south; and such is the real meaning, and the reason 
for the name, of the Compass Plants, of which the most perfect 
and famous example occurs on our own western prairies. In 
some kinds, instead of the leaf-blade it is the petiole which is 
flattened and set vertically, the blade being suppressed, as in 
most of the Australian Acacias (figure 21). In others, advantage 
is taken of the naturally vertical position of the stem, the function 
of foliage being transferred thereto from the leaves which are 
simultaneously reduced or abandoned. This is the case with the 
Cactuses and innumerable other plants of the deserts, which 
sometimes acquire additional vertical green surface by the de- 
velopment of longitudinal ribs. The readiness with which the 
green tissue can be developed in one part of the plant as well as 
another helps, by the way, td explain some of the curious mor- 
phological overturnings represented by plants like the Butcher's 
Broom (figure 23), or, still better, the familiar Smilax of the 
florists, in which the apparent leaves are in reality branches, 
while the actual leaves are no more than tiny scales just beneath 
them. It is easy to understand that if plants of the desert have 
once transferred their chlorophyll to their stems, simultaneously 
suppressing or abandoning their leaves, and then a change of 
climate, or migration to a moister region, should require a larger 
spread of green surface, this would more easily and naturally be 
secured through a further flattening of the stems or branches than 
through a restoration of the lost leaves; and with time such 
branches would become more and more leaf-like even to the 



How Plants Resist Hostile Forces Around Them 265 

extreme degree represented by the Smilax. This very over- 
turning does actually occur in the Cactus family, in which, 
happily, all of the steps without exception are represented by still 
living forms. It is the relics, indeed, of such devious windings in 
the past history of plants which give us our principal morpho- 
logical puzzles. 

This consideration of light naturally suggests the question as 
to heat. This, likewise, is indispensable to plants, since it supplies 
a condition requisite for some chemical reactions and physical 
movements, notably diffusion. Heat also, like light, is more 
and more useful up to a certain intensity (about that of blood 
heat in ourselves), beyond which any increase is not only without 
benefit, but soon becomes an injury. Thus, plants in the fields in 
summer by no means thrive better the hotter it gets. It is doubt- 
ful, however, whether the natural heat of the sun ever attains 
an intensity dangerous to plants, and even if it does, the same 
structural adaptations, especially refractive coverings and a 
vertical position of green tissues, protective against light, would 
be equally effective against heat. And there is perhaps yet 
another method of protection against both, but especially heat, 
namely transpiration, which dissipates through evaporation the 
too intense energy of heat and light thrown into the leaf by the 
sunlight, as we have noted already (page 209). There is, how- 
ever, one place on the earth's surface, and that is in hot springs, 
where low kinds of plants belonging to the Algae can grow at a 
much higher temperature than the sun ever produces, even a 
degree too hot for the hand to endure (up to 81 Centigrade or 
192 Fahrenheit). In these Algae no structural adaptations to 
protection occur (unless a certain slimy coating be such, though 
this is hard to believe), but the living protoplasm has apparently 
become acclimatized to the high temperature, probably by the 
elimination of all chemical constituents affected thereby and the 
substitution of others whose reactions are under full control at 
such temperatures. 



266 The Living Plant 

Thus much for heat; at the other end of the thermometric 
scale more abundant and better marked adaptations are known, 
for the natural temperatures of the earth do fall plenty low 
enough to prove fatal to working plant protoplasm. The first of 
the methods of protection against cold consists in the elimination 
of water, for while moist and working protoplasm is killed near 
the freezing-point, the dry substance can endure temperatures of 
more than 200 Centigrade (over 400 Fahrenheit) below zero 
without injury, and, by the way, in this condition, can also 
endure heat even above the boiling point of water. This power 
of resistance of dry protoplasm against cold and heat is doubt- 
less due to the fact that the injury resulting therefrom is of a 
chemical nature, and the chemical changes in living protoplasm 
proceed only in solution, and solution requires water. 

The protection against cold afforded by dryness explains how 
seeds, which become very dry, can withstand such low tempera- 
tures. Winter buds, however, and the other living tissues of 
plants become only partially dry in winter, and consequently 
are only partially protected by this method; the remainder of 
their safety is probably secured by the slight amount of heat 
released in respiration, which continues all winter, and which 
is effectively conserved by the non-conducting wrappings 
provided in the air-holding bark, and the woolly coatings of buds. 

From these reasonably certain adaptations we turn to some 
others of rather a doubtful sort. The leaflets of many kinds of 
plants, and the flowers of some others, close together or "sleep" 
at night; and Darwin, who studied these movements most 
closely, thought they must form a protection against too great 
cooling at night. This has been doubted of late, and apparently 
with reason, but nobody has given as yet any more probable 
explanation. Again, the red color of the spring vegetation has 
been thought to provide a kind of mitigation of the effects of cold 
weather then frequent, in that by a certain power it possesses of 
converting light into heat, it warms up the parts in the bright 



How Plants Resist Hostile Forces Around Them 267 

but cool days of early spring, when all the warmth procurable by 
the plant is desirable for hastening the development of the 
various parts. This explanation has been applied in particular 
to the red stigmas and styles of wind-pollinated flowers which 
ripen before the appearance of the leaves in the spring, the extra 
warmth thus acquired being supposed to promote the growth of 
the pollen-tube and hence to hasten the fertilization. But here 
we are nearing mere guesswork, and must not accept such sug- 
gestions as explanation, but simply as interesting hypotheses 
deserving of determination through the test of experiment. 

We come now to the most deadly of all the dangers to which 
plants are exposed, and that is dryness As the reader will 
readily recall, water is not only the principal constituent of the 
bodily structure of plants and indispensable in their daily nutri- 
tion, but is also evaporating or transpiring, constantly, co- 
piously, and unavoidably, from all of their younger aerial parts. 
Therefore plants need a constant and uniform water supply, but 
in fact rarely get it, for the most of the kinds, including all of 
those most familiar to us, live under conditions of extreme 
variability not only as to the quantity available for absorption 
by the roots, but also, and especially, as to the quantity forcibly 
transpired from their tissues, these conditions, indeed, being 
linked with the most variable of all things, the weather. Against 
such fluctuations ordinary plants secure a tolerable protection, 
on the one hand through their power of absorbing even the 
hygroscopic water of the soil through their copious root hairs, 
and, on the other, by their complete waterproof epidermis, the 
few necessary openings in which are automatically regulated, 
albeit somewhat clumsily, in adjustment to the prevailing condi- 
tions. But in places where water is permanently scant, as it is 
extremely in deserts, these simple arrangements are insufficient 
and must be supplemented by special protective adaptations; 
and these take three different forms, under which heads we shall 
consider them, (a), increased efficiency of the absorbing system, 



268 The Living Plant 

(b), development of water-storing tissues and organs, (c), ar- 
rangements that minimize loss by transpiration. 

The absorbing system of typical plants, as the reader now 
knows very well, consists principally in the innumerable root 
hairs, which draw water osmotically from a wide area all around 
them. Plants that live in dry places usually exhibit, either as an 
individual adjustment or a structural adaptation, a marked 
intensification of one or more of the features involved in this 
absorption; that is, the number of young roots is larger, the 
hairs are more profuse, the osmotic solutions are stronger, or the 
total range of the root system through the soil is greater. The 
increased profusion of hairs in drier situations is manifest when 
young roots are grown in damp air, where they make a far 
greater display than ever they do in the soil; while the much 
wider range and greater freedom of branching attained by root 
systems in plants that grow in dry places, helps to explain why it 
is that the plants of the deserts are spaced so widely apart, with 
large open areas between them. The presence of stronger osmotic 
solutions inside the absorbing root hairs is distinctive not only of 
some desert plants, but also of others which grow in a different 
situation where water is hard of absorption even though present 
in plenty, namely, in salt marshes, where the water itself is a 
markedly osmotic solution of appreciable strength. As was 
shown in the chapter on Absorption, osmotic absorption by roots 
is dependent on a superiority in strength of the inner over the 
outer solution, and is the/slower and harder the more nearly the 
two approach the same concentration. It is this difficulty of 
osmotic absorption from salt water which explains why large 
vegetation, while crowding as close as it can to fresh-water 
streams and lakes, keeps away from the corresponding situations 
along the margin of the sea. 

The storage of water is the second of the methods protective 
in plants against dryness. All living cells of all plants, indeed, 
possess plentiful stores of water in their sap-cavities, which 



How Plants Resist Hostile Forces Around Them 269 

explains no doubt the reason for the prevalence of the large cell- 
cavity in the construction of plant cells. But many of the plants 
of dry places develop great numbers of specially-large cells ob- 
viously adapted to water storage in particular, and the presence 
of such cells makes the parts that contain them swollen, rounded, 
soft-textured and translucent. This is the origin of the type of 
plant-structure commonly described as succulent, and distinctive 
of many Cactuses, Euphorbias, Mesembryanthemums, House- 
leeks, and others, all of which grow either in deserts, or in other 
places, such as the clefts of rocky hills, where water is scanty for 
long times together. This storage of water is naturally com- 
bined in a great many cases with the storage of food, in which 
case the parts display a firmer texture and whiter aspect in 
section, as illustrated for example by the Century Plants. And 
the examples above given show that the storage organs can be 
leaves, as well as sterns, while roots are frequently used for the 
same purpose. 

The minimization of transpiration is the third and most im- 
portant of the protective adaptations against dryness. We have 
noted already the method by which ordinary plants are protected 
against drought, viz., the possession of a waterproof epidenris 
whose only openings, the stomata, are protectively guarded. 
Now there is apparently no limit to the thickness and perfection 
of waterproofing that can be given by plants to their epidermis; 
and if it were possible for them to exist without the stomata, 
then plants in dry places could wrap themselves up in a way to 
conserve their indispensable water without limit. But as the 
reader well knows, green plants in order to live must have food, 
which is made by photosynthesis, which requires a supply of 
carbon dioxide, which must be drawn from the atmosphere out- 
side. Thus is necessitated the existence of the stomata, which 
must be open for a time and extent directly proportional to the 
food to be made; and this means that water will escape, or tran- 
spire, incidentally but inevitably, through those openings to an 



270 The Living Plant 

amount proportional to the food manufactured. This inevitable 
linking of transpiration with photosynthesis is one of the most 
fundamental of all facts in the economy of green plants. Some 
plants of dry places, especially the deserts, have solved the 
adaptive problem thus presented by condensing all of their 
photosynthetic work into the brief moist season (for all deserts 
where plants can grow at all do possess such a season), spending 
the remainder of the year in a resting and dried state, com- 
parable with that assumed by our plants over winter. But 
others, illustrated by the Cactuses conspicuously, continue their 
activity all through the season, in reliance upon a copious store 
of water and sundry devices for rigid economy of the same. 

This matter of economy in transpiration is so important and 
interesting that we must give it a little further consideration. 
The first and most obvious method thereof consists in the re- 
duction of total green surface, which of course is carried to a 
degree sufficient to keep the unavoidable loss within the limit of 
safety. This is the reason why the plants of dry places, and 
especially of the deserts, are comparatively small, why they are 
so commonly compacted in form, and why they so often are 
leafless, the very object of the existence of the leaf, as the 
reader will recall, being that of spreading more surface. A 
second method consists in the provision of an especially efficient 
epidermis, preventive of transpiration except through the regula- 
ble stomata; and so thick and strong does it become in some 
plants that it is hard to cut and impossible to compress with the 
hands, and actually resembles a coating of horn spread all over 
the plant, as some of the Cactuses illustrate. But a third and 
most interesting method of transpiration economy consists in 
certain arrangements which hinder somewhat the transpiration 
without interfering with the gas diffusion. This to some degree 
is accomplished by a vertical position of the green tissues, for 
while the diffusion of carbon dioxide through the stomata takes 
place with equal facility in any position of the tissues, the tran- 



How Plants Resist Hostile Forces Around Them 271 

spiration is much less from vertical surfaces, since the force of 
the sun which supplies the transpiration energy is obviously 
much less powerful upon vertical than horizontal surfaces. 
Doubtless, by the way, this factor is much more potent than 
those of protection against light and heat in determining the 
prevailingly vertical position of green tissues, whether of stems 
or of leaves, in plants of dry and desert places. And this con- 
clusion is strongly confirmed by the fact that salt marsh plants, 
which need protection against much transpiration though hardly 
at all against light and heat, especially in northern regions, show 
a notable tendency to a vertical position of leaves and other green 
tissues. But the very same end is also attained in a different 
way by the provision, outside of the stomata, of chambers in 
which the escaping vapor is held for a time, thus checking tran- 
spiration a little, somewhat as a damp atmosphere would do, 
while the inward diffusion of carbon dioxide is not appreciably 
affected. In some plants these chambers consist of deep pits in 
the thick epidermis with guard cells lying at the bottom; in 
others the same effect is produced by coatings of hairs or scales; 
while in still others the leaves are inrolled to tubes into which the 
stomata all open. The same result follows, as well, from the 
dense crowding together of leaves, such as desert plants show not 
infrequently (figure 12, center). And many other arrangements, 
notably hardness of tissues, and the presence of gelatinous sub- 
stances, both contributing to water conservation, have been 
described. 

Thus much for dangers from dryness; the other extreme, 
too much water, likewise constitutes at times a danger to 
plants, rarely, however, in any direct manner, but indirectly 
through prevention of the access of air supply. But this matter 
has already been considered along with respiration and aeration, 
where the various protective adaptations (air passages, aerating 
structures, utilization of the dissolved air of the water), have 
been sufficiently described. A very different kind of protective 



272 The Living Plant 

adaptation against damage by water has been claimed by those 
who believe that transpiration is not simply an unavoidable 
evil, but a process of value in itself. The presence of water on 
leaves, derived from dew or the rain, must check transpiration, 
partly by blocking the stomata, and partly by the creation of a 
moist atmosphere around the leaves during its evaporation; and 
any arrangements tending to prevent the wetting, or facilitate 
the drying, of leaves would thus be protective. Such arrange- 
ments do, apparently, exist in those waxy or other unwettable 
coatings which enable leaves to shed water in small drops as one 
can see readily in the Garden Nasturtium, the Pond Lilies, and 
a great many others, some of which show a silvery film of air all 
over the leaf when plunged into a vessel of water. The same 
result is claimed to follow in a different way in those very many 
leaves of tropical rainy regions which .taper off to a long slender 
tip ending in a very small point ; these tips collect, as it were, the 
water-drops as they slip down the hanging leaf, and guide them to 
the point whence their own weight makes them drip to the 
ground, leaving the surface well drained, and ready the sooner 
to begin transpiration. 

There is, however, one way in which water can damage plant- 
structures directly, and it actually happens with grains of pollen 
when these are touched by the rain. The functions and mode of 
growth of these grains, which will be fully described in the follow- 
ing chapter, is such that their walls are necessarily thin and their 
contents osmotically attractive to water, whence it happens that 
if they become touched thereby, they absorb it, swell up, and 
burst, as can be seen very clearly when the water is added to 
grains under the microscope. The pollen, therefore, needs pro- 
tection from rain, whereto a good many adaptations have been 
found, as can be considered, however, more appropriately along 
with the Flower in the chapter devoted to that subject. It has 
also been claimed that the surface-cells of the simple and soft- 
bodied plants of fresh water are subject to a similar osmotic 



How Plants Resist Hostile Forces Around Them 273 

absorption, especially in very warm weather; and this might, if 
too sudden, produce damaging distension and perhaps rupture 
of the walls. But these plants are commonly covered by a thin 
coating of jelly, which is known to greatly impede the rapidity 
of water-passage, thus allowing enough time for an equalization 
of pressures through the stem. This is very likely the adaptational 
significance of the jelly, or slime, of water plants generally. 

But the forces of the air, the earth and the waters are not the 
only ones hostile to plants, for among their worst enemies are 
other plants, and animals. As to plant enemies, the most 
deadly are the parasites, the Bacteria, Molds, Mildews, Blights, 
Rots, Rusts, Smuts, and other Fungi which often destroy their 
host plants entirely. Yet few, if any, positive adaptations have 
been found in plants protective against these parasites, although 
some of the oils and resins occasionally found in leaves do appear 
to afford a moderate protection against Fungi as well as against 
animals. Practically all of these plants reproduce by spores 
which are blown about by the wind; and when these fall upon 
suitable plants they germinate into slender threads. These, 
for the most part, have no power to penetrate the epidermis, 
which is thus someway protective against them; but they enter 
the open stomata and thus reach the soft food-filled cells of the 
leaf, which they proceed to devour. Thus the necessity for the 
existence of stomata involves another danger besides that of 
excessive transpiration, and in this case one against which plants 
seem well-nigh helpless. However, plants differ immensely, 
not only different species but even different individuals of the 
same species, in their susceptibility to injury by parasitic 
Fungi, and there is very good reason to believe that the differ- 
ences have a chemical basis, some kinds possessing a chemical 
constitution hostile to the growth of the parasite while others 
do not. These differences offer a basis for the attempts now 
being made in many experiment stations to combat plant 
diseases by breeding immune varieties, the less susceptible 



274 The Living Plant 

individuals being preserved for breeding in each generation. 
And perhaps it will yet be found that plants can be rendered 
immune by acclimatization, so to speak, to their diseases, as 
animals can be to theirs. But of these matters we know little 
as yet. 

We come finally to the last of the environmental factors hostile 
to plants, and that is the depredations of animals. These, in- 
deed, cannot be otherwise than constant and great, since in the 
long run every jot of the food that is eaten by animals has to be 
ravaged from plants. The general defense of plants, however, is 
passive and indirect, consisting chiefly in a reliance upon their 
own superabundant powers of growth, regeneration, and repro- 
duction, in which features they surpass animals many fold. So 
great are these powers, indeed, that plants are enabled to produce 
organic material in vast excess of their own needs, upon which 
fact depends the very possibility of the existence of animal life. 
And it may be true that the use of this surplus by animals is not 
only of no damage to plants, but may even be useful in removing 
superfluous material and making room for a more active evolu- 
tion of that which remains. In any case it is a generous payment 
for the various services which animals render to plants. 

Although plants thus appear to possess few adaptations against 
animal attacks, especially in their vegetative parts, there appear 
to be notable exceptions. Thus, a good many herbs develop 
various substances in their stems or their leaves, bitter oils, 
turpentiny resins, acrid glucosides, astringent tannins, alkaloids, 
or even needle-pointed irritating crystals, which render those 
plants distasteful to herbivorous animals, including all kinds 
from the greatest of beasts down to slugs and innumerable in- 
sects. Everybody has noticed the clumps of such plants left 
standing quite isolated in pastures by cattle which browse the 
grass well-nigh to the roots all around them. In these cases, 
however, the protection is perhaps incidental rather than adapta- 
tional, and may be defensive against Fungi rather than animals j 



How Plants Resist Hostile Forces Around Them 275 

but there are other instances where the actual adaptation seems 
reasonably clear. Thus, in the desert the conditions of life are so 
hard that plants can scarcely produce any surplus above their 
own needs, while the very precious store of water laid up in their 
stems, and essential to their own safety throughout the dry sea- 
son, is particularly tempting to large animals. In such plants, 
accordingly, we find the best development of features that ap- 
pear to be most protective against animals, either distasteful 
secretions, which are especially common and virulent in plants 
of the deserts, or else a horrid armature of thick-set and dan- 
gerous prickles and spines through which animals can penetrate 
but painfully if at all. Furthermore, a few desert plants are 
known which resemble so closely the background against which 
they grow, cither the rough gray surface of the soil (as in the 
case of the half-buried flat-topped " Living Rock" Cactus of the 
American Southwest), or else the drab pebbles in the beds of dry 
water-courses (as in a number of plants described from the 
peculiar flora of South Africa), that it seems as if such plants 
must surely escape notice by animals, and secure a protection, 
by this form of mimicry, though here again it may be true that 
the result is incidental rather than adaptational. But while a 
protective mimicry seems reasonable in plants of this kind and 
habit, the same can hardly be said of those cases in the flora of 
ordinary climates where some kinds have been claimed to secure 
protection through their resemblance to Nettles, or Poison Ivy, 
or other plants actually noxious. Indeed, so far as concerns a 
protective function for the poison of plants like Poison Ivy, there 
is a difficulty in the fact that the repelling effect is not felt until 
long after the plant has been injured. I think we do not yet know 
the meaning of the poisonous quality of plants. 

An injury done to the vegetative parts of plants does not ex- 
tend to other parts, and is easily replaced; but damage to the 
machinery of growth and reproduction is serious, in correlation 
with which fact we find in those parts a good many apparently 



276 The Living Plant 

protective adaptations. Thus, the preservation of the store of 
food laid up for starting the new vegetation in the Spring is 
obviously of vital importance. We find in fact that the sugar 
and starch stored up by perennial or biennial plants is placed 
underground in bulbs, tubers, or rootstocks, where it is well out 
of the sight and reach of large animals, especially when the 
ground is frozen in winter; while in woody perennials, where the 
food must remain largely above ground, it is scattered thinly 
throughout a large area of tough woody tissue. As to seeds, they 
are often protected by hard coats or extra shells, impervious, for 
the most part, not only to gnawing teeth but also to the digestive 
juices of animals, though in the case of large nuts the teeth of the 
squirrels have won a trifle of the same advantage over the hard- 
ness of the shells that the gunmakers have won over the armor 
makers among our own civilized selves, and doubtless after much 
the same kind of long evolutionary struggle between the two. 
Adaptations to protection of the food supply in these nuts while 
still growing and before the hard seed coats are formed have 
been found in such spines as the Chestnut and Horse Chestnut 
display: in the bitter taste of some pods: and in their green color 
which has been taken for protective coloration, though it is also 
readily interpretable as simply the usual utilization of all avail- 
able surfaces for the spread of more chlorophyll. The greenness 
of edible fruits prior to their ripeness has also been interpreted as 
protective until the time when they turn red or other color and 
aid in dissemination of the seeds, as we shall consider at length in 
the fifteenth chapter. Flowers, likewise, exhibit some remark- 
able adaptations to protection against animals, though in a 
different way, and combined with some more remarkable adapta- 
tions for attracting them, though this is a subject which can be 
considered more conveniently in our chapter on the Flower. As 
to protective adaptations of the growth machinery, which is 
principally the buds, there appear to be several. Not only are 
the buds important as the originators of new growth, but con- 



How Plants Resist Hostile Forces Around Them 277 

sisting as they do of a rich store of succulent food, they cannot 
but prove attractive to smaller animals. Thus, a good many 
kinds, as in the Grasses, place their buds underground, whence 
they send up their stems and leaves to the light. In buds that 
must grow in the air, every soft protoplasmic growing point is 
deeply buried by the leaves it is forming, for these at first lie 
tightly against it, and only later open out to the light. Their 
green color, moreover, must afford an appreciable measure of 
protective coloration, which is doubtless the chief explanation 
of the prevalent greenness of the calyx of flowers. In some 
desert plants, where true leaves are wanting, the buds are sunken 
deep in a hollow formed by the older tissue, and often are further 
protected, as in the Cactuses, by a perfect cheveux de fris of 
tough and interlocking spines. The growing points of roots are 
well protected by their position underground, while the cambium 
cylinder, likewise richly stored with the most nutritious of food, 
is deeply enwrapped by the tough fibrous bark, which often 
contains in addition a great deal of tannin, a substance strongly 
distasteful to most gnawing animals. 

In viewing this list of adaptations, one is constantly reminded 
of the fact that they never are perfect in operation, since animals 
do successfully attack plants against every one of these protec- 
tions. Like so many others of the adaptations of plants, they 
are real and are useful, though clumsy. However, they do obvi- 
ously afford a considerable measure of protection, enough, as it 
seems, to permit plants to hold their own in the struggle; and in 
a world that is full this is all that they need. 




CHAPTER XI 

THE WAYS IN WHICH PLANTS PERPETUATE THEIR KINDS, 
AND MULTIPLY THEMSELVES IN NUMBER 

Reproduction 

[|F all the facts about life, no one is more fundamental 
or familiar than this, that individuals inevitably die. 
Obviously they must be replaced if the race is to con- 
tinue, and this replacement is the office of reproduction, 
which we must now proceed to consider. Our study of the subject 
will have all the more interest for the reason that, like several 
others of the physiological processes, reproduction is substantially 
identical in meaning and method in animals and plants, differing 
only in some external features connected with their differences 
in habit. Therefore any knowledge acquired in one kingdom can 
be transferred to the other; and one may learn from plants the 
essential nature of reproductive processes in animals, including 
mankind. 

The central fact of reproduction is the formation of new in- 
dividuals capable of growing into kinds closely like those which 
produced them. Associated therewith, however, is the further 
fact that usually the formation of a new individual requires the 
cooperation, through the act of fertilization, of two parent in- 
dividuals of different sexes; and so prominent is this feature, 
especially among the higher animals, that most people consider 
it an indispensable feature of reproduction. This idea, however, 
is not correct, and the two things, formation of new individuals 
and sexual union, though so often associated, are quite inde- 
pendent in their nature, as is shown by the fact that purely 
non-sexual, or asexual, reproduction exists abundantly, not only 

278 




How Plants Perpetuate Their Kinds 279 

among plants, but among the simpler animals as well. We may 
best consider this asexual type before proceeding to the more 
familiar sexual kind. 

Asexual reproduction is effected through the separation of a 
portion of plant structure capable of growing into a new plant 
without any preliminary fertilization or other influence of sex. 
In the higher and more familiar plants 
it is rather rare. Almost anybody can 
recall the dark, ovoid bodies which are 
borne by Lilies in the axils of their leaves 
(figure 92), and which easily separate 
and produce new plants. These, though 
seemingly seeds, are really a kind of 
bulblet specialized for this sort of vege- _, M A 

I IG. 92. A portion of stem, with 

tative multiplication; and equivalent leaves, of a Lily, showing the 

ii , , p ., axillary hulblets mentioned in 

bodies are found also on a tew other the text. (Copied from Gray's 
kinds of plants. Again, the runners, Ktruciur<l1 Botan ^ 
suckers, stolons, and other similar structures sent over or under 
the ground by Strawberries, Blackberries, Grasses, and many 
wide-ranging weeds, produce vegetative shoots at their tips, and 
thus propagate vegetatively while securing a kind of dissemina- 
tion, as we shall note more fully in our chapter on the last- 
mentioned subject. It is, however, a fact of great interest, and 
likewise of much practical consequence, that although most of the 
higher plants have lost their power of propagating themselves 
vegetatively, they can yet be made artificially to reproduce in that 
way. Thus, the propagation of plants by slips or cuttings of any 
sort is artificial vegetative reproduction. Great numbers of plants 
will strike root of themselves and grow when slips thereof are 
placed in the earth; others which will make no roots in this way 
can be made to do so by various devices of gardeners; while still 
others which cannot be made to strike roots at all can yet be 
fitted with a set, so to speak, by the operation of grafting, as the 
reader will learn more fully in our later chapter on Growth. 




280 The Living Plant 

Asexual reproduction in the lower and simpler plants takes 
place in two principal ways. In the tiniest plants, which com- 
prise the Bacteria, or Germs, and some of the simplest Seaweeds, 
the entire plant body consists of no more than 
a single cell, which in reproduction splits di- 
rectly across the middle; the two parts then 
grow promptly to full size, only to split again, 
and so on without limit (figure 93), a method 
called fission, or division. Nothing could be 
simpler, which explains the extreme rapidity of 

Fia. 93. Stages in the . , . ' . . , _ , - A i 

division of a, one- multiplication in the forms that possess it. And 

rolled plant, Pleuro- , , , . i ,1 , ji ji 

coccus highly magni- ^ 1S interesting to observe that this is exactly 
ficd. The ceils later fa e method whereby single cells reproduce in 

fall apart. (Copied J & i- 

from the Chicago even the highest of plants. A second method 

Textbook) 

of asexual reproduction in the simpler plants 
consists in the formation of asexual spores, which, under great 
diversity of habit and form, exhibit these features in common, 
--that they are single cells separated off from the parent plant, 
and capable of growth directly each to a new plant. In some 
Seaweeds they are provided with swimming appliances whereby 
they can move through the water in a manner so suggestive of 
animals that they are known scientifically as zoospores (figure 
94), though in other Seaweeds 
they are drifted passively about 
by the water-currents. Non- 
motile spores occur in many 
land plants; are formed in pro- 

fusion in the gills of Mushrooms, FIG. 94. Forms of free-swimming repro- 

tho o<mmilp<* of ductive cells of which the two on the left 

Tile CapSUies OI 





arc ascxua i Z0 6spores, while the two on 

Spots (or SOri), On the the right are sexual cells, later to be de- 
t ^ " scribed. 

under sides of the fronds of 

Ferns, and in the black stalked heads that develop on various 
Molds (figure 95), whence they are wafted on the wings of the 
wind to the uttermost parts of the earth. Oft-times these asexual 




How Plants Perpetuate Their Kinds 281 

spores are provided with coats of such thickness and hardness as 
to make them immune against every natural condition of dry- 
ness and heat for weeks, months or years together; which fact, in 
conjunction with their power of floating with the dust in the air, 
explains their ubiquitous penetration into 
all kinds of strange places. It is thus 
that Bacteria and Yeasts, for example, 
are enabled to spread so widely as they 
do. 

Such is asexual reproduction, which 
never involves fertilization, and has no 
relation whatsoever to sex. In sharp 
contrast stands sexual reproduction, in- FIG. 95. The spore caso of a 

i ,1 .. ,. ,1 * / ,.,. Mold, discharging itn asexual 

volving the cooperation, through fertihza- sporos; hiKhly maKn ified. 
tion, of two parents which are usually, |^ y B r o!d r Picd pic " 
though not always, of different sexes. 

We may best begin our study of fertilization with the more famil- 
iar plants, where it occurs in the flower, which is a structure 
specially adapted thereto. In a typical flower, as everyone 
knows, the outer green protective calyx and the inner colored 
showy corolla together enclose the stamens and the pistils (fig- 
ure 96). The stamen consists of a slender stalk crowned by a 
chamber, the anther, containing a fine yellow dust, the grains of 
pollen, inside of which develop the male cells of the plant. The 
pistil consists of a rounded chamber, the ovary, extending upwards 
into a lengthened stalk, the style, ending in a roughened swelling, 
the stigma, and containing one or more ovules in which develop 
the female cells of the plant. If, further, a critical examination is 
made of a typical ovule, by aid of longitudinal sections and 
microscope (figure 97), it is found to enclose inside of some coats 
a definite cavity, the embryo sac, within which in turn is consider- 
able protoplasm and several cells, including a larger one close to 
the end where an opening (the micropyle) is left through the coats. 
This larger cell is the female generative cell, the exact equivalent 



282 The Living Plant 

of the egg in animals, and hence called the egg-cell Thus much 
for the female reproductive apparatus; turning now to the male, 
we find it in the pollen grain, which, examined microscopically, 
is found to consist of at least two cells (figure 98), of which one 

gives rise to the male, or 
sperm, cells, presently to be 
further described. Such is 
the floral structure, and such 
the appearance and locations 
of the sexual cells, when fully 
ripe and ready for fertiliza- 
tion. 

The process of fertilization 
itself can be followed in de- 
tail by aid of the microscope, 

Fit*. !)0. Interior view of a typical flower (of . . . 

Peony), showing the four distinctive parts, - and IS sllOWIl 111 CSSCntials by 

the outer calyx, made up of sepals, the more v j i / 

showy corolla, made up of petals, the many Ur generalized drawing (fig- 




stamens, and the two pistils, which in this 11T .~ QQ\ TVr* r*ai u+rvr* ic r^rJ 

i , . | i . i HI. v/ tJ t.7 1 JL 11\> IIJ-DL OLiVyL/ lo IJLJJL 

rase show typical ovaries, but very short 7 x x 

styles and small stigmas. (Copied from lination, or the transfer of the 

Strasburgcr's Textbook). . 

pollen grain from the anther 

to the stigma, and since the pollen is usually brought from a sepa- 
rate plant, the process is far more elaborate than one would 
imagine, and one, withal, which involves so many striking and 
interesting features that we must treat the subject in a chapter 
by itself; and that chapter follows. When the pollen grain 
reaches the stigma, to which it is held by a certain roughness 
aided by a sugary stickiness, it immediately begins to send 
out a slender tube. This tube, which carries in its tip two 
nuclei that represent the essential parts of two male cells, grows 
down into the tissues, through which it dissolves its way 
by aid of enzymes secreted by the tip, the dissolved substance 
being absorbed for food; and thus the tube literally digests its way 
down through the tissues of stigma and style to the cavity of the 
ovary. Here it passes out from the solid tissue, and, guided as it 



How Plants Perpetuate Their Kinds 



283 



seems by some chemical vapor exuded from the micropylar open- 
ing of the ovule, grows straight thereto and enters, pursuing its 
way until its tip comes to lie flat against the egg-cell. Then one 
of the male nuclei moves out of the tube into the egg-cell (fig- 
ure 100), and across it to the 
nucleus thereof; then the two 
nuclei touch, flatten a bit 
against one another, and 
finally fuse and intermingle 
completely. Thus the egg- 
cell comes to possess a nucleus 
made up from the two nuclei 
derived from the two parents, 
male and female; and this is 
the central and most essen- 
tial feature of fertilization. 
The fertilized egg-cell is now 
ready to grow into an em- 
bryo, which, with certain ac- 
cessory parts, forms the seed, 
and later grows to an adult 
new plant. 

I have described somewhat 
fully this process of fertiliza- 
tion as it occurs in an ordi- 
nary plant because it is typi- 
cal in principle of all fertilization through the plant and animal 
kingdoms. The machinery varies immensely of course in detail. 
In some kinds of plants the sperm cell is not carried by a growing 
tube, but, guided by certain attractive chemicals exuded by the 
female parts, swims of itself in water to the egg-cell, as is the way 
in Ferns, Mosses and many Seaweeds. In most animals, how- 
ever, the male cells (called spermatozoids) are brought by suitable 
organs to the near vicinity of the egg-cells, to which they finally 




FIG. 97. A typical ovule (of Narcissus), seen in 
optical longitudinal section, highly magnified. 
The parts may be identified from the text, 
the most important being the egg-cell, which 
is the larger of the three cells lying in the 
upper ond of the embryo sac. (Copied from 
a wall-chart by Dodel-Port). 



284 The Living Plant 

swim of themselves. But in all cases the principle is the same; by 
suitable structures and accessory adjustments and adaptations, 
the male cell is brought into contact with an egg- 
cell, to which it passes over its nucleus, with more 
or less cytoplasm; the two nuclei then fuse, and 
thus is formed a fertilized egg-cell from which the 
new individual develops. 

In following this process it becomes evident that 
the object of all the elaborate machinery of fertili- 
FI. 98. A typi- zation is to secure the union of the male and female 

cal pollen grain , . f ., , . ., , i u 

(of Tradescan- nuclei, for that is the one feature which is com- 




ticai ^ctl and pk*dy constant throughout. This of course raises 
highly magni- the question as to what the nucleus actually is. 

fied, showing, on 

the loft, the cell As our chapter on Protoplasm showed, every nu- 
tho" ! \na UC 'or cleus contains a certain peculiar matter called 



(Ooried f r^ni chromatin, which, ordinarily scattered throughout 
strasburger's the nucleus, collects itself at times into a definite 

Textbook). 

number of rod-like structures called chromosomes 
(figure 101). The evidence seems to show beyond question, 
though the method thereof is in doubt, that these chromosomes 
embody within themselves the characteristics of the parent 
plants, (or, constitute the working plans or patterns thereof, 
so to speak), and in such manner as to exert control over the 
development of offspring, and ensure that new individuals shall 
grow up in resemblance to those that produced them. Now 
in fertilization each nucleus contributes its chromosomes, so that 
the nucleus of the fertilized egg-cell contains a double number 
derived equally from both parents (figure 100). The significance 
of this fact, however, becomes apparent only as we follow the 
behavior of the chromosomes during the development of the 
fertilized egg-cell into an adult plant; for in such development, 
the egg-cell first divides across into two, and then its parts into 
two, and so on until the whole plant is completely grown. Now 
in the first division each one of the chromosomes, both those 



How Plants Perpetuate Their Kinds 285 

derived from the male and those derived from the female parent, 
split lengthwise into two, and one of the halves goes into one new 
cell and the other into the other; and they absorb nourishment 
and grow with the cell. This process is then repeated with every 
subsequent division, so that finally every 
cell of the adult plant contains chro- 
mosome material derived from each one 
of the parents of that plant. This fact 
helps to explain how it is that a plant or 
an animal can resemble either one of its 
parents in any detail of its structure. 

At this point I will pause for a moment 
to consider two matters of minor impor- 
tance which may have occurred to the 
reader. If the fertilized egg-cell contains 
the sum of the chromosomes of the two 
uniting nuclei, why is not this number 
doubled again in the next generation, 
and that again doubled in the next, and 
so on to their enormous multiplication? 
The explanation is simple; at some period 
in the development of the new sexual 
cells, by a method which we need not FI. OO.-A generalized drawing 
here trace in detail, the number of of a ? im p le ? va ?7 " nd Y ul 

J seen in longitudinal section, 

Chromosomes is reduced to One-half. In showing the parts concerned 
,, , , . f n , . in fertilization. 

the second place, if every cell contains 

within itself chromosome matter derived from both parents, and 
therefore has the possibility of resembling either one of its parents 
in any detail of its structure, what is it that determines which one 
it shall resemble? This we do not yet know, but the probability 
would seem to be that in each case the stronger of the two elements 
overpowers the other and reproduces its like. 

The equal contribution of chromosome matter by male and 
female nuclei, together with the subsequent regular splitting of 




286 The Living Plant 

each chromosome in cell division, carries an implication of great 
importance to an understanding of the nature of sexual reproduc- 
tion, namely, it implies that both parents contribute exactly alike 
to the characteristics of the offspring, the selection between the 
double set of paternal and maternal characters being made in the 
course of development of the offspring itself. This view is dia- 




Fio. 100. A diagrammatic representation of fertilization, showing the passage of the male 
nucleus from the pollen tube into the egg-ceil, and its fusion with the nucleus thereof. 
The black rods in the nuclei are chromosomes, described on page; 284. 

metrically opposed to the older idea, once advocated by some 
biologists, that each parent contributes something the other does 
not; and it is obviously quite different from the various popular 
notions, which, naturally, are largely erroneous. 

But now there arises this fundamental question. If the two 
sexes contribute substantially alike to the offspring, why are they 
not substantially alike in structure? What is the meaning of the 
differences between the sexes? Or, to go a stage deeper, why does 
sex exist at all? Happily these questions can be answered with 
reasonable certainty through evidence supplied by a study of 
existing transitions from the simplest plants, where sex has not 
yet developed, to the highest plants and animals, where it is fully 
differentiated. Thus, there exist some simply-organized seaweeds 
which throw out into the water a great many reproductive cells, 
all exactly alike and provided with suitable structures for swim- 
ming (figure 102). These move towards one another and come 
together in couples, which then fuse completely, uniting their 
nuclei; and thus is formed a " fertilized 77 cell which gives origin 
to a new plant precisely as does a fertilized egg-cell. Obviously 
fertilization in this case occurs between sexual cells precisely alike; 



How Plants Perpetuate Their Kinds 



287 



or, if one pleases, it is fertilization without sex. In the next place 
there are other and more highly organized seaweeds in which the 
reproductive cells given off into the water are of two different 
kinds, although produced by the same parent plant. One kind is 
very much larger, round, and without arrangements for locomo- 




FHJ. 101. The appearance and behavior of the chromosomes during the division of a 
typical plant cell, as seen, somewhat generalized, in a series of optical sections highly 
magnified. A fuller description of the division of the chromosomes is given on page 
284 of this book. (Copied from Strasburger's Textbook.) 

tion, while the other is much smaller, of elongated shape arid pro- 
vided with good swimming organs (figure 103). All of the move- 
ment necessary to bring the two cells together in fertilization is 
made by the active smaller cell, which, guided no doubt by some 
chemical secretion, swims to the passive larger one and fuses 
therewith; the two nuclei then unite and from this fertilized cell 



288 The Living Plant 

a new plant is developed. Now the difference between the two 
cells is known to consist in this, that the larger possesses a store of 
food substance, which is used in giving a start to the new individ- 
ual, the presence of this food substance in the cell being re- 
sponsible both for its larger size and its loss 
of locomotive power. The smaller cell, on 
the other hand, contributes no food for start- 
ing the offspring, but elaborates the features 
concerned in locomotion, thus ensuring that 
the two cells shall be brought together. This 
^ii" difference does not imply in the least that 
(of the water net) the two cel i s contribute differently to the 

highly magnified. At ^ 

the left is a single one offspring, for the food substance supplied by 

and at the right a pair 

in process of fusion, the larger cell has no more to do with de- 
termining the essential characters of the new 




individual than has the food we eat in 
determining our essential characters; the characters are de- 
termined by the chromatin in the nuclei, which the two cells 
contribute equally. But in this comparatively minor feature of 
division of labor between the two kinds of cells we have the origin 
of sex, for the larger cell we recognize as female and call it the egg- 
cell, and the smaller we recognize as male and call it the sper- 
matozoid (or antherozoid). This difference between a large 
immobile egg-cell and a tiny active male cell, once established, 
persists and prevails in principle, though with numerous varia- 
tions of detail, throughout all of the more highly organized plants, 
and throughout all of the higher animals, inclusive of man; and it 
is the foundation of all the phenomena of sex. 

The essential characters of the sexual cells being thus estab- 
lished in these comparatively low plants in a degree of develop- 
ment as high as they ever attain, the sexual developments in the 
higher plants are concerned not with the sexual cells, but with the 
various accessory structures contributing to ensure fertilization 
under the conditions to which those plants are exposed. A first 



How Plants Perpetuate Their Kinds 



289 



step in the development of such secondary sexual structures is 
found even in the higher Seaweeds (the Red Algae), in which the 
egg-cell is not cast loose, as in the lower forms, but remains at- 
tached to the parent plant that forms it and supplies its store of 
nourishment; while simple arrangements exist to facilitate the 
access of the male cell. ' But far more important is the step taken 




FIG. 103. A scries of figures illustrating the reproduction of the common Rockweed. In 
the middle lower part is the spherical female (egg) cell, highly magnified, surrounded 
by a number of the very much smaller free-swimming male (sperm) cells. 

in the simplest land plants, like the lowly Liverworts and Mosses, 
and the fertilization stage of the Ferns, i. e., a stage in which the 
plant is a tiny thin leaf pressed close to the ground. In all of these 
plants the very delicate egg-cell and the subsequent embryo need 
protection from the dryness to which they must occasionally be 
exposed, and a protection, of course, which does not interfere 



2 go 



The Living Plant 



with a ready fertilization. The structure which has been devel- 
oped in adaptation to these conditions is in form of a flask-shaped 
covering to the buried egg-cell (figure 104) ; the end of the tube 
opens when the egg-cell is ripe and water is present, and exudes 
a special liquid chemotropically attractive to the spermatozoids, 




L'ICJ. 11)1. A series of figures illustrating the. reproduction of a common Fern. The sexual 
cells are borne on the under side of a small thin leaf-like part close to the ground. 
In the lower middle part of the picture is a squarish egg-cell with prominent nucleus, 
buried in chlorophyllous tissue, and covered with an elongated-tubular structure, 
down the cavity of which a spiral-shaped male cell is proceeding to unite with the 
egg-cell. 

which then swim towards and down the neck to the egg-cell. But 
such plants are as obviously dependent upon water for fertiliza- 
tion as are the Seaweeds; and hence they are confined to places 
habitually wet, or must grow so close to the ground that fertiliza- 
tion can be effected during flooding by rains. Still another step, 
but this time the final one for plants, in the evolution of secondary 



How Plants Perpetuate Their Kinds 291 

sexual structures, is taken in the flowering plants, which carry 
their sexual parts high up in the air. In consequence of the greater 
dryness of that situation, they have had to bury their sexual parts 
far more deeply (viz., deep inside the ovules and anthers), and 
have had to abandon the free-swimming sperm cell of all the 
lower kinds and replace it by the growing pollen-tube, which 
carries the male cell to the female cell in the way we have already 
described. In a word, the structures developed adaptively in re- 
sponse to the conditions of protection and fertilization in these 
highest plants are the stamens and pistils, the essential parts of the 
flower which we have already described. But all such structures, 
like all of the sexual parts and adaptations developed by animals, 
are in reality secondary, being merely arrangements to enable the 
male cell to effect fertilization and the female cell to receive it. 
The central and essential feature of fertilization and sexual union, 
viz., the union of nuclei carrying the hereditary qualities of two 
parents; and the central and essential feature of difference between 
the sexes, viz., a division of labor between the two parents; 
these remain the same throughout the plant and animal kingdoms 
from the lowliest of the seaweeds up to man himself. 

Sex, therefore, does not arise in any essential difference of rela- 
tion of the two parents to offspring, but in a minor and mechanical 
matter of division of labor between the sexual cells, involving 
secondary differences in various accessory structures. Sex, so to 
speak, is not a matter of method but of mechanism, and exists not 
for the sake of the formation of offspring but for giving it a more 
certain and better start in its life. That cell, structure, or in- 
dividual, which is devoted to nourishing and protecting the young 
individual formed as a result of fertilization we call female: that 
cell, structure, or individual, which is devoted to bringing the two 
cells together for fertilization we call male. This difference is the 
central feature of all the phenomena of sex, although worked out 
with infinite variety of detail and more or less interlocked with 
other considerations; and it explains not only sexual structures in 



292 The Living Plant 

plants, but in animals also, including man. With man, indeed, 
the principle that the female is the receptive and protective 
element, and the male the aggressive element, is not limited to 
physical structure alone, but shows its influence in some of the 
profoundest facts of his actions, thoughts, laws, and social cus- 
toms. 

At this point the reader may demur to this explanation of sex 
on the ground that it seems too superficial for the profundity of 
the phenomena. But one should take care not to extend to all 
Nature conceptions derived alone from mankind, where all sexual 
matters are vastly exaggerated in apparent importance by socio- 
logical considerations. In Nature at large sexual differences are 
prominent rather than profound. Even in plants that are highest 
in the evolutionary scale, sexual differences never affect the plant- 
structure very far away from the pistils and stamens; and all of 
the remainder of the bulk of the plant has nothing whatever to do 
with sex, but is strictly nonsexual or asexual. Even the occasional 
cases, described by the term dioecious, where one plant bears only 
pistils and another only stamens, is no real exception, though we 
often describe these plants, very naturally, as male and female 
respectively. The individual, therefore, in plants is sexual only 
in limited spots, it is never sexual as a whole. The same is true 
of the simpler animals, but in the more highly organized species 
it is somewhat different, for in them each individual bears only 
one kind of sex cells, and has only one kind of sex organs; while 
the high specialization of these parts affects somewhat the whole 
individual so that we distinguish male and female individuals. 
But even in mankind the structural differences between the sexes 
are insignificant as compared with the structural resemblances 
between them. 

Whatever else my discussion of sexual reproduction may have 
meant to the reader, it will at least have demonstrated this, that 
sexual reproduction is a far more complicated process than 
asexual, involving the construction and manipulation of adapta- 



How Plants Perpetuate Their Kinds 293 

tions wholly needless in the asexual methods. Yet plants can 
reproduce perfectly well by the direct and simple asexual methods. 
In what, then, consists the superioritj^ of sexual reproduction that 
plants and animals should not only take so great trouble to secure 
it, but should even abandon in the higher forms the asexual 
method entirely? Many answers have been proposed for this 
question, and we do not yet know the truth with certainty; but 
the most probable explanation is derived from the fact that sex- 
ually produced individuals are usually more variable, adaptable, 
and vigorous than those asexually produced, and hence in the 
long run overcome the latter in the struggle for existence, and sur- 
vive while the others die out. An asexually generated individual 
is naturally no more than a chip of the old block, and can differ 
but little therefrom, while one sexually produced has the possi- 
bility of combining the good qualities from two parents. Now if 
conditions surrounding plants were unchanging, and all plants 
were adapted the best possible thereto, then the asexual method 
might be really the better; but the conditions of the world are 
continually changing, and therefore those animals and plants 
which possess the most variability and adaptability have the 
best chance of maintaining themselves therein. This is the reason, 
I think, why sexual reproduction, despite its complications, has 
displaced the far simpler asexual kind. 

The greater constancy of characteristics usually distinctive of 
asexual reproduction, in comparison with the greater variability 
associated with the sexual type, has some very practical conse- 
quences. Thus, as everybody knows, we can reproduce Bartlett 
pear trees by the asexual method of grafting, and keep the fine 
qualities of the fruit; but if we propagate them by seeds, which 
of course are formed only as a result of fertilization, we do not get 
Bartlett pears at all, but just a plain mongrel variety; and the 
same thing is true of many other kinds of highly perfected garden 
plants. This principle is so well understood by gardeners that 
whenever they seek to secure new formfc of plants, they make use 



294 The Living Plant 

of seed propagation; and whenever they have obtained a specially 
good kind, they try to preserve it by propagating it asexually. 
But we are verging over to the subject of plant breeding, which 
is a matter so important that it must later receive a chapter to 
itself. 

We must here turn back to fertilization in order to consider 
another important phenomenon in connection therewith. Al- 
though in the higher plants both pistils and stamens are usually 
associated closely together within one flower, it is only excep- 
tionally the case that egg-cells are fertilized by pollen from that 
same flower. On the contrary there exist the most elaborate ar- 
rangements adapted to prevent such a close fertilization, and 
ensure that the sex cells which unite shall come from different 
flowers, and usually indeed from different plants. It is in adapta- 
tion to such cross pollination that plants have developed the more 
conspicuous features of the flower, the nectar, odor and showy 
corolla in particular, as will appear in the following chapter which 
is wholly devoted to this interesting subject. Indeed, in some 
plants the arrangement is such, notably where the stamens and 
pistils are borne on quite different plants, that close fertilization 
is not even possible; and this arrangement is universal among the 
higher animals. Now it is quite plain that the fertilization of an 
ovule by pollen from the very same flower would be vastly easier 
of accomplishment than is the elaborate cross fertilization, re- 
quiring no more, indeed, that a simple turning of the stamen over 
against the stigma, when the growth of the pollen-tube would 
accomplish the rest; and the fact that plants not only choose the 
most difficult method, but also even abandon in the higher forms 
the very possibility of the simpler, shows quite conclusively that 
cross fertilization has some great merit above close fertilization. 
And the reason for the superiority is not difficult to find. It was 
first indicated by the experiments of Darwin, who showed that 
the progeny resulting from cross fertilization can be more vigorous 
and numerous than those from close fertilization; and the same 



How Plants Perpetuate Their Kinds 295 

thing is confirmed in general by the experience of animal breeders 
who know that the parents must be not too nearly related if the 
offspring are to be of the best. Indeed, it is evident that if pollen 
and ovule belong to the same flower or even the same plant, we 
have a near approach to vegetative reproduction; while the full 
value of fertilization can be realized only when the uniting sex 
cells come from different individuals. In a word, cross fertiliza- 
tion has much the same advantage over close fertilization that 
fertilization has over asexual reproduction; and this advantage 
has sufficed to enable the kinds which have developed it to 
triumph over those which have not. If it seems to the reader that 
in cases like these Nature goes to a trouble out of all proportion to 
the advantage attained, I would remind him that life is a kind of 
race in which only a few can be winners; and that no effort can 
be too great to put forth when to live is the prize and to lose is 
death. 

There is, furthermore, still another matter of the highest impor- 
tance which must receive our attention in connection with ferti- 
lization. Everybody has heard something about Mendel's Law, 
though it is not, as yet, widely understood. Mendel was an Aus- 
trian monk, who, in his monastery garden, a half century ago, 
began experimenting systematically upon heredity, and thereby 
discovered the most important facts we yet know about that fun- 
damental subject. In order that the characters transmitted by 
each parent might be distinguishable in their offspring, he selected 
as parents not plants of the same variety, but of distinct varieties 
differing markedly in some given features; and furthermore, in 
order to avoid the complications caused by cross fertilization, he 
chose kinds which fertilize themselves, as do a number of culti- 
vated species. Accordingly, taking Peas, which fulfil these con- 
ditions, he bred together a kind with green cotyledons and an- 
other with yellow cotyledons. The resulting offspring were of 
course hybrids, but their cotyledons were not, as one would 
expect, greenish-yellow, or yellowish-green, but were all yellow 



296 The Living Plant 

like those of one of the parents and quite free from the green of 
the other. These hybrids, when grown, were also self-fertilized, 
and produced a large number of offspring; and as a result a re- 
markable fact came to light, namely, that although approxi- 
mately three-fourths (75%) of these plants possessed yellow 
cotyledons, one-fourth (25%) had green cotyledons just like those 
of one of the grandparents. Furthermore, when these green- 
cotyledoned forms were propagated by self-fertilization, all of 
their numerous offspring had green cotyledons, and never yel- 
low, which color was thus permanently bred out of these plants 
and their descendants. But when the 75% of yellow-cotyledoned 
forms were self-fertilized, their offspring gave this striking result, 
that one-third of them (and therefore one-fourth, or 25%, of the 
entire original number in this generation) produced only yellow- 
cotyledoned kinds, as did their offspring, and theirs again indefi- 
nitely, the green being thus bred permanently out of these forms 
and their descendants. But the remaining two-thirds of the yellow 
kind (forming half, or 50%, of the total number), acted, when 
self-fertilized, precisely as their parents had done, producing 25% 
green, and 75% yellow kinds of which latter 25% bred perma- 
nently yellow; and the same thing was repeated in the next genera- 
tion, and so on without limit. The method of this distribution of 
characters in the offspring is shown graphically in the accom- 
panying diagram (figure 105), in which, however, the exigencies 
of printing forbid the use of color, for which reason the yellow 
cotyledons are represented by the solid black circles, and green 
by the white ones. Moreover, the first generation, (and half 
of the later individuals) though themselves possessing yellow 
cotyledons as a visible or so-called dominant character, have 
obviously the power of transmitting the green as an invisible or 
recessive character; and this fact is represented in the diagram by 
giving to those individuals a white center. It is not yet wholly 
clear, by the way, why Peas which have the power of transmitting 
both yellow and green cotyledons should always have yellow ones; 



How Plants Perpetuate Their Kinds 297 

but such is the fact, and such dominance of one character over 
another is always a feature of Mendelian inheritance. 

Now this remarkable distribution of contrasting characters is 
true not of the cotyledon color of Peas alone, but of their flower 
colors, their height, and other characteristics; and not of Peas 
alone but of innumerable other plants, and likewise of the most 
diverse animals, in the most diverse characters. Moreover, while 
discovered and most conspicuous in hybrids, it is also true in prin- 
ciple of all kinds which breed together; and while its mathe- 
matical basis can be traced clearly only in self-fertilizing forms, 
it holds true, though of course with proportional complications, in 



9 






rn rn i i i i i i i i rn i i 
oooo ooo* ooo* 

FIG. 105. A diagram illustrating Mendelian inheritance. It is fully explained in the text. 

cross-fertilized forms. There is, indeed, no longer any doubt that 
it represents a very wide spread principle of heredity. Indeed, 
were it not for the numerous complications introduced by the 
complexity of life-phenomena, it would probably be found to hold 
true universally. 

Mendel's discoveries have thus shown not only that heredity is 
correlated with a certain mathematical principle, but also that 
any undesirable feature can be rapidly and surely bred out of a 
race, and need not require the slow process of dilution out, so to 
speak, as was formerly supposed. 

There is one fact about this Mendelian distribution of characters 
which will illuminate the whole subject greatly if the reader will 



298 The Living Plant 

but grasp it clearly at the start, namely, that it applies only to 
single individual characters, never to large collections of them, 
and much less to the whole aggregate of characters displayed by 
each parent. Each of the many characters transmitted by parents 
to offspring conforms to this general principle, but they are 
transmitted to no two of the offspring in the same combinations. 
The offspring are thus like the different patterns displayed by the 
same pieces of colored glass in the turning of a kaleidoscope; and 
this is very well exemplified in the familiar cases among mankind, 
where the characteristics of two parents reappear in the most 
different combinations in their children. Nevertheless, in self- 
fertilized races of plants it is possible to fix permanently certain 
combinations of characters by breeding out their opposites, and 
then these combinations repeat themselves with the greatest 
fidelity. Such combinations we shall meet again in our chapter on 
evolution under the name of genotypes. 

Finally, we consider for a moment the explanation of the re- 
markable mathematical arrangement revealed by MendeFs Law. 
In the first place it is plain that each definite character of the 
adult individual is represented by some kind of determiner in the 
germ cells (i. e. egg-cells and sperm cells), and that any individual 
is a mosaic of characters of which the germ cells (or, rather, their 
chromosomes), are collections of the corresponding determiners. 
Also, the facts show that, in harmony with the behavior of 
the chromosomes in cell-division and fertilization, while every 
body cell of each individual contains two determiners for each 
character (one derived from each parent), every germ cell, because 
of the reduction division earlier mentioned (page 285), contains 
only one or the other of these determiners and never both, a fact 
expressed in the phrase " purity of the germ cells." Thus in our 
Peas, earlier instanced, each of the male cells, and also each of the 
female cells, contain a determiner for either a yellow or a green 
cotyledon, but never both. Now if large numbers of such male and 
female germ cells are allowed to come together at hap-hazard, as in 



How Plants Perpetuate Their Kinds 299 

fact they do in fertilization, then a certain proportion of those con- 
taining yellow determiners will unite with others containing yellows, 
and green will be excluded from the resulting individuals; a certain 
proportion of greens will unite with greens, thus excluding yellow; 
a certain proportion of greens will unite with yellows, and a cer- 
tain proportion of yellows will unite with greens. These propor- 
tions (allowing for the fact that the yellow-green and the green- 
yellow are indistinguishable) will be precisely those actually 
found by the law to exist, as above described, and precisely those 
shown in our diagram. 

There remain a few miscellaneous matters, connected with 
reproduction, which must be considered before this chapter can 
be brought to a close. 

It is almost invariably the case that an egg-cell must be fer- 
tilized by a male cell before it will grow to a new plant, but a few 
exceptions are known. In some few plants of the Composite 
family, and in the Plant Lice among Insects, the egg-cells grow 
directly into new individuals without any fertilization or other 
connection with the males, a phenomenon appropriately called 
parthenogenesis. It is a kind of asexual growth of the egg-cell, 
comparable with the growth of a tiny bud; and possibly the es- 
sential meaning of the process lies in its asexual character. It is 
conceivable that these particular kinds of plants and animals have 
reached the highest practicable stage of adaptation to the con- 
ditions around them, in which case it would be natural for them 
to preserve their characteristics unchanged by resorting to asexual 
propagation, using the method which entails least disturbance to 
existent structures and habits. 

In my account of fertilization I showed that the pollen grain 
when it enters the embryo sac contains two separate nuclei, only 
one of which is needed to fertilize the egg-cell. The fate of the 
other is most peculiar, for in some plants at least, and probably 
as a rule, it fuses with a nucleus belonging to the embryo sac 
itself. The resultant cell grows into the mass of food substance 



300 The Living Plant 

(the endosperm) , devoted to nourishing the embryo in its growth. 
At first sight it would appear as if the embryo and the endosperm 
were brothers, so to speak, one of which later feeds cannibalis- 
tically upon the other; but probably this is not its meaning. It is 
more likely that the fusion is simply a method of providing a 
stimulus for the formation of the endosperm; a signal, so to 
speak, to the waiting embryo sac nucleus that fertilization has 
really been accomplished and therefore the endosperm will be 
needed, for the endosperm does not form unless fertilization is 
accomplished. The matter, however, would have a purely 
scientific interest were it not for a rather well-known phenomenon 
it explains. Most people are aware that some varieties of corn 
produce red ears while others have white ones, and that some- 
times, where the two kinds grow together, red grains appear on 
the white ears. This has long been known to be due in some way 
to the influence of the pollen of the red kind upon the white ears, 
but the remarkable matter about it was this, that the color was not 
in the embryo, where its presence would be natural, but in a part 
of the grain which was apparently made by the white parent. 
Here was a case in which the male parent not only fertilized the 
egg-cell, but even seemed to affect the structure of the female 
parent, a phenomenon called xenia in plants, and often reported, 
though never confirmed, among animals. But the discovery of 
this double fertilization removed all mystery from the matter, for 
the color in the red grains resides wholly in the endosperm, which 
is a kind of a hybrid between the male and female parents, sharing 
in the characters of both. 

As our chapter on Protoplasm showed, individuals tend to wear 
out and die when their protoplasm repeats longtime the same 
function; but they can live potentially forever if the protoplasm 
can change periodically its internal arrangement, can go, so to 
speak, into the melting pot, and be cast anew. Now there is no 
more effective remelting than that accompanying sexual reproduc- 
tion, for a greater change in the constitution of the protoplasm 



How Plants Perpetuate Their Kinds 301 

could hardly be imagined than that which occurs through the 
commingling of two different cells. At all events fertilization is 
always followed, especially in animals, by that display of vigor 
and activity which we call youth or juvenescence, whereby the 
racial vigor is periodically renewed in each generation. Indeed, 
so prominent and advantageous is this rejuvenescence that some 
biologists have thought to find therein the chief utility of sexual 
reproduction. Perhaps it does indeed play some part, for sexual 
reproduction, like many other physiological processes, is probably 
not the expression of a single factor, but the resultant of the co- 
operation of several. 

Replacement of the individuals which must die is no doubt the 
first meaning of reproduction, but therewith is often associated 
the idea of multiplication in number. Multiplication, however, 
is more seeming than real, as shown by this fact, that in general 
any kind of animal or plant, no matter how numerous its off- 
spring, does not alter its numbers appreciably from one year to 
another. Thus, in general, there are no more Mushrooms, Dan- 
delions, or Robins in a given county this year than last, and the 
numbers of each kind remain for decades substantially stationary. 
Even the occasional exceptions caused by the introduction of new 
weeds or animal pests, or by the expansion of man himself, is no 
real exception, for after a time these also attain a condition of 
numerical stability. Hence, the offspring formed by animals and 
plants do not in general increase their numbers, but simply make 
up for losses. In reproduction, therefore, multiplication is sub- 
ordinate to continuance of the kind. 

Reproduction, as we have seen, is essentially Nature's method 
of continuing the kinds of plants or of animals as the individuals 
perish. This being true it follows that if the individuals were 
immortal, there would be no need for reproduction, after once 
the world was fully populated. This view receives confirmation 
from the balance which exists between the vegetative prosperity 
of the individual and its reproduction, anything favoring the one 



302 The Living Plant 

tending to check the other. Thus, many simple forms will not 
form reproductive parts so long as the solutions in which they live 
contain plenty of food, and the other conditions are favorable; 
and it is only when they begin to feel the effects of insufficient 
food or temperature that they will begin to form reproductive 
bodies at all. Even in the higher plants the same principle holds, 
and all farmers know that when soils are too heavily fertilized 
many plants tend to "run to leaf/' and flower very badly, while 
there are plants of our greenhouses (e. g. Bougainvillaea) which 
must actually be partially starved before they will form any 
flowers. The same principle holds good with animals; they must 
not be too highly pampered and fed, else their reproductive powers 
suffer. I believe that we have the operation of the same principle 
upon a very large scale among mankind in the fall of the birth-rate 
amongst the most highly civilized races, and the highest classes 
of each race. In general the birth-rate is lowest where the hygienic 
and other conditions are most favorable for the preservation and 
comfort of the individual, and the birth-rate grows higher among 
peoples and classes in which the conditions of life are markedly 
harder. Harder conditions of life presage an earlier end to the 
life of the individual, and Nature seems to have adopted their 
presence as the stimulus or signal for setting the reproductive 
apparatus more actively at work. 




CHAPTER XII 

THE MANY REMARKABLE ARRANGEMENTS BY 
WHICH PLANTS SECURE UNION OF THE SEXES 

Cross pollination; Flowers 

|HE preceding chapter should have made it quite clear 
that plants possess sex; that this is the same, both 
female and male, as it is among animals; that a union 
of the two is generally needful for the production of 
offspring; and that the offspring is usually better in quality if the 
uniting sex cells are derived from separate parent plants. But 
the union of sex cells from separate parents presents a difficult 
problem to those plants which, including all of the higher and 
more familiar kinds, are sedentary, and therefore unable to come 
together by their own powers of locomotion, as animals, and indeed 
some of the water plants, so readily do. Specifically, their prob- 
blem is this, to secure the transfer of the small and light pollen 
grains, which contain the male cells, from the anthers of one 
plant across some space to the stigmas, which give access to 
the female cells contained in the ovules, of another, after which, 
of course, fertilization proceeds by the methods already described 
very fully in the chapter on Reproduction. The problem of cross 
fertilization, therefore, resolves itself in such plants into one of 
cross pollination, which is effected by methods that we must now 
consider in detail. 

Let us first dispose of the simpler methods displayed by the 
Water plants, which in some cases possess an animal-like power 
of independent locomotion by swimming, particularly in their 

303 



34 



The Living Plant 




male cells. In most of the Seaweeds (or Algae), of both salt and 
fresh water, both kinds of sexual cells are cast out into the water, 
where those from different plants become completely com- 
mingled, especially under action of currents, waves, and the 
power of the male cells to swim freely 
about; and apparently mere chance under 
these conditions is enough to ensure a 
sufficiency of crossing between different par- 
ents, although, for all we know, elaborate 
physiological arrangements, comparable 
with some of those which will presently 
be described for the higher plants, may 
exist to prevent union of sex cells pro- 
duced by the same plant. Such arrange- 
ments, indeed, are known to occur in the 
higher kinds of plants fertilized in the 
106. Ceils, from dis- water, notably the Ferns, where the male 
and female sex cells Produced on the same 
lant ripen at different times. Again, in 

. . 

(From the Chicago Text- some other kinds of low Water plants, whose 

book ) 

habits are such that the many long threads 
of which their bodies consist live tangled or felted together, slender 
tubular projections (a kind of premonition of the pollen-tube), 
grow out and connect one thread with another (figure 106) ; and 
through the passage thus formed the contents of one cell can 
unite with another in cross fertilization, though plenty of cases 
are known in which the same method is used in the fertilization of 
one cell by another within the same thread. 

While the Seaweeds, or Algae, are the distinctive plants of the 
waters, a good many kinds of Flowering plants, originally in- 
habitants of the land, have been forced into life in the water, 
developing, of course, appropriate adaptations thereto. Of these, 
the conspicuous kinds, like the Water Lilies, secure their cross 
pollination by the very same methods as the showy-flowered 



Arrangements for Securing Union of Sexes 305 



plants of the land, which we shall consider a few pages later; but 
a great many others of simpler sort, including especially the 
lowlier Waterweeds, cast their suitably-protected pollen out into 
the water, to be drifted about by the currents until it reaches the 
stigmas. In some kinds, as in 
most of the Eel-grasses, where 
the pollen is thread-like in 
shape, the pollination occurs 
under water; but in others, for 
example the Freshwater Eel- 
grass, Vallisneria spiralis (figure 
107), it takes place on the sur- 
face, to which the staminate 
flowers rise from their place of 
formation, and on which floats 
the ripe ovary with widely- 
spread stigmas. Then the 
movements of the surface cur- 
rents, with aid of the wind, 
bring the pollen sooner or later 
to the stigma. 

But far more striking and 
important are the adaptations 
to cross pollination found in 
plants that live out on the land, 
including the kinds with which 
we are most familiar. These, 
having no power at all of loco- 
motion, have had to secure the 
transport of their pollen in some different way and that way con- 
sists in the utilization, by aid of suitable adaptive mechanisms and 
methods, of such motive agencies as happen to exist in the world 
around. Now of all such agencies, the most ubiquitous and the 
easiest to utilize is the wind. Accordingly wind pollination prevails 




Fia. 107. Cross pollination in the Water- 
weed, Vallisneria spiralis, which is shown 
about one-third the natural size. The 
staminate flowers may be seen rising to 
the surface, where they open and are 
drifted about until their stamens come 
into contact with the long-stalked float- 
ing pistillate flowers. (Copied, somewhat 
simplified, from Kerner's Pflanzenleben.) 



306 



The Living Plant 



in a good many plants, especially trees, and, in lesser degree, 
shrubs, for these are most exposed to the sweep of the winds; while 
it is rare in herbs and confined mostly to those that grow in fully 
exposed places. In such plants the smooth light pollen grains often 
possess bladders, or wings providing more surface for action of the 
wind (figure 108), while, moreover they are produced in vast 




FIG. 108. Typical pollen grains, highly magnified. On the left next above the bottom 
row, are three from the Pine, showing the attached bladders. The very rough kinds, 
especially those of the upper row, arc carried by insects, to whose hairy bodies they are 
thus adapted to cling. (Reduced from Kerner's PJlanzenleberi) . 

quantities to compensate for the inevitable waste inseparable 
from this method. For this reason the staminate blossoms of such 
plants far outnumber the pistillate, as witnessed by the fact that 
long hanging staminate catkins, from which one can dislodge a 
cloud of fine yellow dust by a touch, are familiar to everybody in 
Birches, Alders, Poplars, Butternuts, and other trees in the 
spring; while the pistillate blossoms, which commonly occur on 



Arrangements for Securing Union of Sexes 307 



separate plants, or at least in separate 
flowers, are comparatively so incon- 
spicuous that they scarcely are known 
at all, and need a considerable search to 
reveal them. The relative conspicuous- 
ness and abundance of the two kinds of 
blossoms are typically shown by the 
Hazel (figure 109). When found, how- 
ever, these pistils are distinguished by 
large, and usually branched or hairy, 
stigmas, an obvious net spread for the 
stoppage of the wind-drifted pollen. 
Thus the "silk" of the Corn, wherein 
each strand is a style along which grows 
a pollen-tube to each grain, stands out 
from the young ears when their grains 
are ready for fertilization, as a feathery 
cluster of styles and stigmas, which 
catch the pollen carried by wind from 
the staminate tassels, though later when 
its usefulness is past, the silk withers 
limply down. In ca>ses where no stigmas 
are present, as for example in many 
cone-bearing plants, like the Spruces and 
Pines, there is usually some arrangement 
of smooth scales which guide the inci- 
dent pollen down to the vicinity of the 
ovules. Furthermore, it is obvious that 
the efficiency of wind pollination de- 
pends on the greatest possible freedom 
of wind action through the branches, 
and therefore on absence of interference 
by the leaves. This is the reason why 
so many wind-pollinated flowers open 




FIG. 109. Flower clusters of the 
European Hazel, a typical wind 
pollinated plant, showing the 
great disproportion in bulk be- 
tween the male and the female 
flowers, the former being the 
long drooping catkins, and 
the latter the small ovoid- 
tufted structures. (Copied 
from Kerner's Pflanzenleben.) 



308 The Living Plant 

in the very early spring before the leaves have appeared, as 
catkins for example all do, and the flowers of some Maples; 
while that first feathery bloom shown by Elms against the 
spring sky is caused by the wind-pollinated flowers, and not 
by the leaves as most folks think. The same end is attained 
in a different way in those cases where the blossoms are borne 
out at the extreme tips of the branches, as in most kinds of 
evergreens, while a still more notable example is found in the 
Grasses, which raise their spikes or panicles of inconspicuous 
greenish blossoms high over the leaves, as any meadow well 
illustrates. And a good many other adaptations to wind pol- 
lination are found in particular cases. But in general these 
features, occurrence on trees in particular; light and super- 
abundant pollen, and therefore relatively prominent male blos- 
soms; much-branched stigmas on prominently placed though 
rather inconspicuous female flowers; an early blossoming period 
or an exposed blossoming position distinguish the wind-pollinated 
plants. And to these characters may be added another, of a 
negative though no less distinctive sort, that such flowers possess 
hardly any of the features that we commonly associate with the 
name, no bright colors, aside from an occasional case of the 
early spring red, no odors, no nectar, no striking forms, no great 
size. The reason for their absence is obvious enough, such 
features are not needed in wind pollination. 

But wind pollination, widely used though it is, becomes almost 
insignificant when compared with a different method which sur- 
passes it many fold in economy, efficiency and extensiveness of use. 
A great disadvantage of wind pollination consists in its waste- 
fulness; for of all the great quantities of pollen cast out on the 
winds from the anthers of plants, not more than an insignificant 
proportion can happen to fall on receptive stigmas. One can 
gather, indeed, a vivid idea of the wastefulness of this method from 
the fact, which some of my readers may have seen for themselves 
as I have, that in northern countries, where wind-pollinated 



Arrangements for Securing Union of Sexes 309 

trees, especially the cone-bearing kinds, are particularly abundant, 
the little lakes of the woods are covered in the spring time with 
pollen enough to make a continuous film all. over the surfaces, 
while of course an equal amount must fall on the land. So plenty 
at times is the pollen in the air of those countries that it receives 
the expressive appellation of " sulphur-shower. " Now pollen, 
composed as it is almost wholly of the richest protoplasmic ma- 
terial, is one of the most difficult and expensive of substances 
for plants to manufacture; and therefore the wastefulness of wind 
pollination must entail a great drain on these plants. Obviously, 
any method which would ensure the transfer of pollen direct 
from the anthers of one plant to the stigmas of another would 
be greatly superior in both economy and certainty to wind pol- 
lination. Such a method, indeed, plants have developed; and it 
consists in the utilization of the locomotive powers of animals, 
especially insects. 

We turn, accordingly, to the study of the cross pollination of 
flowers by insects. Obviously a first requisite of the method is an 
arrangement that will lead insects to go directly from flower to 
flower, a thing which they will not do unless induced by some 
attraction or compulsion. The inducement takes the form of a 
store of nectar, a sugary liquid both nutritious and palatable 
to insects, and easily made by plants in little superficial glands. 
These nectar glands, which often pour their product into special 
receptacles called nectaries, and which exhibit a great variety of 
forms in different flowers, are of course placed in close juxtaposi- 
tion to the stamens and pistils (figure 110). They constitute the 
most fundamental feature of insect-pollinated flowers; and those 
plants which possess them along with stamens or pistils but no 
other parts, for example the Willows and some Maples, represent 
a first stage in the evolution of the insect-pollinated flower. But 
a second requisite of the method is some arrangement by which 
the position of the inconspicuous nectar (and therefore of the 
stamens and pistils), can be made evident to the insects; and this 



3io The Living Plant 

is accomplished by the provision of a blotch of color, which is 
formed and spread in a special set of leaves developed for the pur- 
pose, the corolla. This is the reason for the existence of color in 
flowers; it is a notice or signal, advertising to insects the position 

of the nectar, which is the real at- 
traction. Finally, a third requisite 
of the method is such a construction 
of the flowers as will make it inevi- 
table that the insect, as it enters a 
pollen-ripe flower in the quest for its 
nectar, shall receive on its body a 
supply of the pollen which it will as 
inevitably leave on the stigma in 
entering an ovule-ripe flower. And 
this is the explanation of the princi- 
pal peculiarities of shapes and sizes 
in flowers, which, because insects are 
most diverse in form and habits, 

FIG. HO.-A flower, enlarged, of the are themselves equally diverse in de- 
Rape with petals and sepals re- fai\$ o f construction. Furthermore, 

moved to show the contiguity of 

the nectar glands (the ovoid struc- it is plain that the reason for the 

tures near the base) to the stamens . ., i j-i 

and pistils. (Copied from Goebd's separation of the stamens and pistils 
outlines of classification.) into separate flowers in the wind- 

pollinated kinds does not hold in those that are pollinated by 
insects; for in these, on the contrary, there are advantages, as to 
economy of number of blossoms and also of insect visits, in hav- 
ing both stamens and pistils associated in the same flowers. This, 
accordingly, is the prevailing condition in showy blossoms. 

Thus it is evident that the most striking features of the flowers 
of the higher plants, including the ones with which our very con- 
ception of the flower is most closely associated, the colored 
corolla, nectar, odor, and striking peculiarities of shapes, exist in 
adaptation to cross pollination by insects. Or, the matter can be 
stated in this way, the flower is an organ evolved in adaptation to 




Arrangements for Securing Union of Sexes 311 

the advantage of the cooperation of two parent plants in the produc- 
tion of offspring. 

In my discussion of this subject I am assuming that the reader 
already has some general knowledge of the relationship existing 
between flowers and insects. Surely 
there is no one whose attitude towards 
nature is such as to lead him to read 
thus far in this book, who has not ob- 
served with interested attention the 
actions of insects among the flowers 
in a garden ; and a little more watch- 
ing will always reveal the same 
things in the flowers of field, road- 
side or forest. But it may be well if 
I insert at this point, in further illus- 
tration of our subject, a description 
of some conspicuous examples of ad- 
aptations to cross pollination. 

There grows commonly in Europe, 
and sparingly in this country where it 
has been introduced, a small upright 
herbaceous plant called Aristolochia 

FIG. 111. Flowers of Aristolochia 
whose yellow tubular Clematitis, just before and just 




blossoms, an inch or so long, stand 

upright and invitingly open when ^pimed somewhat from Sachs' 

ready for fertilization. It is cross 

pollinated, by small flies, which, bringing pollen on their bodies 

from other flowers, slip easily down the tube through the 

downward-pointing hairs (figure 111). Then, working around 

after the nectar in the middle part of the chamber, to which 

they are confined by other hairs in the base, they leave their 

pollen on the stigmas (the hooked structures of the figure), 

which soon curl back out of the way of further pollination. Im- 

mediately the hairs in the base wither up, and the insects go there 



312 



The Living Plant 




for the nectar, when the anthers open and shed new pollen on 
their bodies. Then the nectar-secretion ceases, and simulta- 
neously the hairs in the throat, hitherto impassable in an up- 
ward direction, wither up, and the insect hies him away with his 

load to another one of the 
flowers. Finally the flower be- 
comes partially closed at the 
mouth, as the second figure 
shows, and droops on its stalk; 
then it is sought no more by 
insects, whose visits would ob- 
viously be useless. 

Another common European 
field plant, sometimes seen in 

FIG. 112. A Salvia flower (substantially like OUr gardens, is that Mint 
8. prafansiti), in general view and in sec- 11 j cr 7 / 

tion, showing the mode of cross pollina- Called balVia pmtenSlS, 

tion described in the text, The line on the i ne h-lono- bright blllP 

sections indicates the direction of thrust of 1I1Cn 1On & DHgni-DlUe, 

the insect's proboscis. (Copied, with slight zontally-set, irregular flowers 

simplification, from the Chicago Textbook.) 

possess stamens remarkably 

hinged on their stalks (figure 112). These stamens are con- 
structed on the principle of the lever, with the long arm 
carrying the anthers up inside the upper lip, and the short arm 
resting down like a valve over the entrance to the nectar tube. 
The cross pollinators are bees, and when one of these insects, 
coming to a pollen-ripe flower, alights on the lower lip, which is 
suitable in size, form and position for its reception, it pushes its 
head into the tube for the nectar and thus forces back the short arm 
of the lever, which, swinging on the intermediate hinges, brings 
down its longer pollen-laden end on the back of the bee in just the 
position where that insect is struck by the overhanging stigmas 
as it enters another flower that is ready for fertilization. 

One of the most wide spread of American Orchids is the little 
wood-dwelling Habenaria orbiculata, which sends up a long loose 
cluster of greenish-white flowers from two glossy round leaves 



Arrangements for Securing Union of Sexes 313 



spread flat on the ground. The flower, which is shown greatly 
enlarged in the accompanying picture (figure 113), has a struc- 
ture so remarkable that without elaborate observational studies 
no one could ever imagine either the identity or the use of the 
parts. But the strap-shaped piece in 
front is a petal; the opening at its top 
leads into the greatly-elongated nectar 
tube shown next behind it ; the two struc- 
tures converging above this opening are 
the halves of one anther, each of which 
contains a great many pollen grains tied 
together into one mass by threads; these 
threads collect together into two sticky 
discs shown as two white oval structures 
each side of the opening; and the darker 
space between anthers and opening is the 
stigma. The reader will readily recog- 
nize how different is this construction from 
that of an ordinary flower; and the im- 
plication that the parts must possess un- 
usual functions is correct. The cross- 
pollinating insect is a moth, with a 
proboscis (ordinarily carried in a pendant F IO . 1 13. Flower, much 
close coil) having a length sufficient to 
reach to the bottom of the nectar tube 
(figure 114). It alights upon the strap- 
shaped petal, whose narrowness compels 
its approach in a very definite position, 
and, as it pushes far down for the nectar, 
it brings the two sides of its head, its huge eyes, to be exact, 
into contact with the two sticky discSj which come away with 
their attached pollen as the insect withdraws. Moths have 
often been caught with these pollen masses attached to their 
eyes, which were formerly supposed to be afflicted by some kind 




larged, of an Orchid, Habe- 
naria orbiculata, cross polli- 
nated by the remarkable 
method described in the 
text. The hindermost part, 
not there mentioned, is the 
ovary and stalk. (Reduced 
from Gray's Structural 
Botany). 




314 The Living Plant 

of strange parasite. Almost immediately, during the flight of 
the insect from flower to flower in fact, these pollen masses 
droop on their stalks, and hang down in such position that when 
the insect probes into a new flower they do not strike the anther 
but are pressed down directly on the sticky 
stigma, which holds them tenaciously. And 
thus is cross pollination effectively per- 
formed. 

These three cases have been chosen not 
because they are especially remarkable, but 
because they illustrate several different 
features of cross-pollination methods. In- 
deed, the number of equally-striking cases 
^ legion, requiring whole volumes for their 
pollinates the ffabenaria adequate description; and many of the ar- 

of figure 113; the pollen * . 

masses are attached to rangcments might well stagger belief were 

its eyes. (Reduced from . f n n , , , . . , 

Gray's structural they not fully confirmed by the critical 
Botan y^ studies of large numbers of competent in- 

vestigators. But while we cannot take space to describe any more 
individual cases, for which the reader, if interested, may turn 
to the works described in the footnote,* we must follow some- 
what farther a few of the matters brought up in the foregoing 
discussion. 

* The principal works upon cross pollination likely to prove of interest or use to 
the reader are the following: The foundation of all is Sprengel's book entitled (in 
translation, for the work is in German) Nature's Secret displayed in the Construction 
and Pollination of Flowers (1797), a classical work a half century ahead of its time, 
and a treasury of accurate information on its subject, though it mussed, of necessity, 
the central illuminating idea of the value of cross as compared with close pollination, 
Next in importance came three of Darwin's greatest books, The Various Contrivances 
by which Orchids are Fertilized by Insects, The Effects of Cross and Self Fertilization in 
the Vegetable Kingdom, and Different Forms of Flowers on Plants of ttie same Species, 
which three works contain a greater amount of new observation and illuminating 
explanation than any others we possess. The first general summary of the entire 
subject was Miiller's Fertilization of Flowers, a translation of a German work, which 
is admirable in all respects, and superseded only by the cyclopedic work by Knuth, 
Handbook of Floral Pollination, just completed, likewise a translation from the 



Arrangements for Securing Union of Sexes 315 




In the first place what is it which prevents close pollination in 
flowers where both sexes are present? Against this obvious 
difficulty, however, floral evolution has made ample provision. 
The simplest method is a physiological one, viz., a flower is 
sterile to its own pollen, that is, 
a given stigma will not permit the 
growth of its own pollen thereon, 
doubtless for some chemical 
reason; while another phase of 
the same thing is the fact true of 
some plants, that if close and cross 
pollen happen to fall simultane- 
ously on a stigma, the cross pollen 
is the one that grows fastest and 
produces the fertilization. But 

FIG. 115. Dichogtimor.s flowor of Clero- 

f ar Commoner IS the Simple and daidron, on two successive days, show- 

., nc j.' j f i ing the different time of ripening of 

perfectly effective device Of mak- sta mens and pistils. (Reduced from 

ing the stamens and pistils of <****'* structural Botany.) 
each flower ripen at different times, an arrangement called 
dichogamy (figure 115), and found in a good many common 
plants. Again, close pollination is prevented by mechanical 
arrangements, usually the interposition between anther and 
stigma of some specialized outgrowth, as shows veiy well, for 
example, in the common Blue Flag, or Iris (figure 116). Still 
another arrangement is displayed by the Primroses, Bluets, and 
Mayflowers, which possess two kinds of flowers bearing stamens 
and pistils in different positions, with corresponding differences in 

Gorman. The best general account of the subject, admirably written and beauti- 
fully illustrated, is contained in Kerncr's Natural History of Plants (another transla- 
tion from the German), while his smaller volume, Flowers and their Unbidden Guests, 
is a charming presentation of that subject. Brief and popular summaries have been 
given by various writers, notably by Asa Gray in his all too brief How Plants Behave, 
by Lubbock, in his Flowers, Fruits, and Leaves, and by W. IT. Gibson in his Blossom 
Hosts and Insect Guests, which is the most readable of all the works on the subject. 
All of these books should be found in the public libraries. 



The Living Plant 



pollen and stigmas (figure 117); a plan of structure called 
dimorphism. When the suitable insect visits in succession several 
flowers of the different kinds, it receives pollen on its body from 
the upper stamens in a position to leave it on the tall stigmas, and 

the same for the shorter kinds; 
while any accidental pollination 
of a stigma from the same or a 
similar flower produces no effect, 
because of the differences in pollen 
and stigmas aforementioned. 

But although such elaborate ar- 
rangements exist in adaptation to 
the prevention of close pollination, 
in other kinds of flowers there are 
features which as obviously secure 
it. Thus, in a great many of the 
simpler and regular kinds of flow- 
ers, the pollen falls normally on 

FIG. 116. An Iris flower in partial sec- JT j p . i n 

tion. Above the stamen is a petaloid the Stigmas ot the Same flower, 

and P roduces close fertilization in 




from the stamen, is stigmatic. (Copied case no CrOSS pollen is received, 
from Gray's Structural Botany.) .. 

though if cross pollination does 

occur, then cross fertilization is effected instead. But a much 
more extreme case is found in those flowers which never open 
at all, and in which the pollen-tubes grow out from the 
anthers to the immediately contiguous stigmas, and thence 
effect fertilization in the usual way (figure 118). Such flowers, 
called cleistogamous, lie close to the ground, and are well known 
in Violets and some kinds of Oxalis; but this fact is conspicuous 
about them, that the same plants in all cases possess also the 
ordinary showy kinds of blossoms cross pollinated by insects. 
Obviously, therefore, cleistogamous blossoms, like the cases of 
close pollination earlier mentioned, represent a method of en- 
suring close fertilization in case a cross should happen to fail, 



Arrangements for Securing Union of Sexes 317 




on the principle that although cross fertilization is better than 
close, a close fertilization is better than none. And for this 
principle there is much other evidence. In all of these cases, 
however, at least an occasional cross fertilization must be ef- 
fected; and there is good reason to be- 
lieve that while many kinds of plants 
can endure close fertilization for a con- 
siderable time, they must have an oc- 
casional cross in order to retain their 
full vigor. 

But we must turn for a moment to 
view cross pollination from the side of 
the insect. Our discussion thus far may 
have seemed to imply that insects Fl( < . m.-Djmorphous flowers. 

t enlarged, of Partridge Iscny, 

exist in Certain Sizes, forms, and habits, further explained in the text. 
1T ., i i i^ix (Reduced from Gray's titruc- 

nxed by other considerations, and that turai Botany). 
the adaptations between them arid 

flowers have been wholly effected by modifications of the flow- 
ers. This, however, is not correct, for there is every evidence 
that in the course of evolution, insects have become adapted 
to flowers as well as flowers to insects, as indeed we might expect 
from the fact that while it is an advantage for flowers to have 
their pollen carried by insects, it is an advantage to insects to be 
able to obtain their food from the flowers. There are, of course, 
many kinds of insects which never visit flowers at all; and it is 
only the kinds which are nectarivorous, so to speak, that plants 
have been able to provide an attraction for, and only these kinds 
show adaptations to flowers. 

For the success of cross pollination of flowers by insects, it is 
obviously essential that the insects shall habitually visit plant 
after plant of the same kind, rather than first one kind of plant 
then another, which happen to blossom together. For no result 
follows a cross pollination between different kinds. Observation 
always shows that in fact insects do as a rule visit the same kinds 



The Living Plant 

of plants successively, as anyone can see for himself in a garden; 
while experiment indicates that they are primarily .guided by 
color, which, probably, in their equivalents for minds, becomes 
associated with flowers in which the nectar is ready. This process 




FIG. 118. A plant of the common Blue Violet, displaying the contrast between the 
familiar showy flowers and the cleistogamou.s kind, which are the bud-like struc- 
tures on the recurved lower stems. At the right is a clcistogamous flower in section, 
showing the contiguity of anthers and stigma. (Copied from Atkinson's Textbooks.) 

is greatly aided in nature by a correlative peculiarity in the plants 
themselves, namely, that different kinds of flowers which 
blossom together at the same time are usually strongly contrasted 
in color, as any meadow, or brookside, or autumn roadside il- 
lustrates. It is true that the insects do not visit only one flower 



Arrangements for Securing Union of Sexes 319 

on a plant and then visit only one on another, as would be theo- 
retically the best of arrangements; for on the one hand that were 
too difficult a thing for the plant to be able to induce the insect 
to do, and on the other it is needless. What happens in reality is 
this, that an insect in visiting a plant usually goes successively to 
all the flowers that are open, and thus becomes thoroughly dusted 
all over by a mixture of pollen, which is ample in quantity to 
allow some for each stigma of all of the flowers on the next plant 
that it visits. Of course there is mixture of pollen, and a great 
deal of pollination between different flowers on the same plant; 
but the method makes probable the presence of some cross pollen 
on each stigma, when the selective power of the stigma for cross 
pollen, already mentioned, ensures cross fertilization. And the 
matter is aided a good deal by a peculiarity of blossoming which 
practically all plants show, that no large number of flowers are 
open at one time in the same cluster, no more, one may say, 
than as many as an insect can pollinate by the quantity of pollen 
it can carry on its body from a previously-visited plant. Of 
course none of these arrangements are exact in their working, but 
are general, or average, or clumsy, with many individual failures. 
But on the whole they suffice. 

That insects find flowers chiefly through the colors seems un- 
doubted, but there is more in the subject than appears at first 
sight. The chief essential of floral color, from this point of view, 
is conspicuousness, which of course involves contrast with the 
background; and as this is commonly green, therefore white and 
yellow and red are the commonest of floral colors, especially in 
flowers that nestle among foliage. The less contrasted blue is 
rather more common in flowers that stand out by themselves, 
whether singly or in long terminal clusters. Furthermore, it is 
true that some kinds or groups of insects show preference for 
certain floral colors; and, correlatively, the flowers having such 
colors are prevailingly of a size and construction better fitted to 
the visits of those insects than of others. Thus, most small and 



320 The Living Plant 

regular open flowers are yellow or white, and visited by a great 
variety of small insects, especially flies. Blue flowers, however, 
are visited mostly by bees, and, as the Larkspur and Monkshood 
well illustrate, possess in general a position of nectar, a compul- 
sory mode of access thereto, and an arrangement of stamens and 
stigmas such that bees can best of all insects get the nectar and 
most surely carry the pollen. Red flowers, such as the Pinks, are 
oftenest visited by Butterflies, whose probosces are long enough 
to reach to the bottom of their slender tubes for the nectar which 
is there inaccessible to the very much shorter probosces of Bees. 
Again, white flowers, in highly specialized kinds like the Orchids, 
are preferred by Moths, which are indeed the only insects possess- 
ing probosces of a length sufficient to reach to the bottoms of the 
unusually long nectariferous tubes. The reason of course why 
the insects prefer the respective colors is because these have 
come to be associated with a construction of flower from which 
they can easily draw nectar, while that nectar is pretty sure to be 
present because other kinds of insects are largely excluded. These 
relations, as before, are not precise in detail, but operate as a 
general principle; and, as a general principle, also, it is true that 
insects, floral colors, and floral structure have evolved together in 
harmonious correlation. 

While considering this subject of floral colors, I may here add 
a number of miscellaneous matters of particular interest. Thus, 
as to white color, it is found to distinguish most flowers that bloom 
in the dusk of the evening, that being of course the one color 
which is most conspicuous in darkness; and such flowers commonly 
exhibit the very long nectar-tubes and other constructional 
features adapting them to the visits of moths, which are chiefly 
night-flying in habit. This is the explanation of the peculiarities 
of the Night-blooming Cereus, Nicotiana, and some Jessamines. 
Quite a different aspect of floral conspicuousness is involved in 
the brilliant coloration of flowers that grow in rather inhospitable 
places, such as Arctic shores, Alpine heights, and desert wastes. 



Arrangements for Securing Union of Sexes 321 

Alpine plants in particular are famous for their beautiful colora- 
tion. An explanation thereof has been found in adaptation to the 
comparative scarcity of insects in these places, the extra brilliancy 
representing the extra difficulty of ensuring their visits. Again, 
a good many flowers exhibit a considerable variegation of color, 
consisting chiefly of definite spots or lines quite different in hue 
from the ground color of the flower as a whole, as Forget-me-nots 
and Nasturtiums well illustrate. But these markings are found 
always to have one feature in common, that they indicate the 
position of the nectar. The floral color as a whole brings the 
insect to the flower from a distance, and these markings then show 
it the place to probe for the nectar, which of course brings it 
into the position where it can best leave its pollen and receive an 
additional supply. Again, the effectiveness of color is obviously 
increased by massing, which explains the value of clusters of 
flowers, especially for kinds that are small. Finally, as to this 
matter of color, we need note but one more peculiarity. Some 
kinds of flowers, though none that are very familiar, change color 
immediately after fertilization; and it is claimed that such flowers 
are no more entered by insects, whose visits would obviously be 
useless to both the flowers and themselves. The same end is here 
attained, though by a different method, as in the case of the 
drooping flowers of the Aristolochia already described. The ad- 
vantage to the species as a whole of preventing useless visits of 
insects, and thereby conserving their services for flowers which 
still need them, is sufficiently obvious. 

As an advertisement to insects of the position of the flower, 
color often is aided, and sometimes replaced, by odor. It has 
even been claimed in late years that insects are guided to flowers 
much more by odors than colors, many of such odors being hardly, 
or not at all, perceptible by us; but the evidence on this point has 
not yet won acceptance. However, there is no question at all as 
to the assistance rendered by odor to color in those cases where 
color alone cannot be made sufficiently conspicuous. This is true 



322 The Living Plant 

especially of night-blooming flowers, in which the association of 
sweet odor and white color is very common. This same aid of 
odor to color is found in those flowers which bloom in very in- 
conspicuous positions, such as close to the ground, or among 
leaves in the shade, as the Mayflower illustrates; and in general 
odoriferous flowers that are not night-blooming are the shy little 
kinds of the woods. Odor also aids color, or acts as a substitute 
in some flowers which have not attained to a corolla, or have lost it, 
as in some Willows and Maples. On the other hand, flowers that 
grow in exposed places, and display an abundance of color, very 
rarely possess any odor, as the tall kinds of the meadows, the 
river-banks, the autumn roadsides and the prairies all illustrate, 
the absence of sweet flowers from the prairies in particular being 
matter of common knowledge and frequent comment. And 
finally, as to odor, we need note but one more point, that while 
most floral odors happen to be pleasing to us, there are some that 
are not, as in case of the Skunk Cabbage and a good many others 
of that family. But such odors have their lovers among insects 
to which they are doubtless more sweet than all of the spices of 
Araby. Indeed, it is only a fortunate accident that any of the 
odors of plants .give us pleasure at all ; for in their evolution our 
tastes in the matter were not in the least consulted. 

Color and odor suggest nectar, which is the real attraction to 
insects in the great majority of flowers. It can usually be seen 
very easily at the bottoms of floral tubes where it lies as a clear 
watery liquid; and sometimes in special receptacles of more open 
flowers it stands out in great glistening drops, as conspicuously 
illustrated by the Crown Imperial. However, a good many 
flowers are without it entirely, in which case the attraction is 
pollen, then produced in unusual abundance; for some insects 
prefer pollen to nectar, making use of it not only for food, but 
also for building their honeycomb cells. And if the reader should 
ask me why some flowers use nectar while others use pollen as 
their means of attraction, I agree that I will tell him when he has 



Arrangements for Securing Union of Sexes 323 

told me why one man is a carpenter and another a farmer; or why 
the Latin races are artistic while the Teutonic are practical; or 
why the Germans are the best scientific investigators in all the 
world. 

The symmetry of my subject would seem to demand that I add 
to these paragraphs on color, odor, and nectar, another devoted 
to the mechanical arrangements in flowers in relation to cross 
pollination. But I despair of giving any adequate idea of this 
subject in the space that remains at my command, and it must 
suffice to say that such arrangements are both remarkable and 
innumerable, involving not only the most extreme modifications 
in all of the parts, but such special features as sensitively-bend- 
ing stamens (in the Barberries), closing stigmas (perhaps, in 
Mimulus), springing stamens (as in Mountain Laurel), explosive 
stamens (as in Mallows), forcibly-projected pollen-masses (as in 
some Orchids), and others as striking, which the reader may 
follow as far as he pleases through the many good books devoted 
to the subject. 

It is doubtless sufficiently obvious why insects are the animals 
most used for cross pollination by plants, for their small size, 
active flight, and especially their nectarivorous habits, make them 
especially available for this purpose. But it must at the same 
time be remembered that those very features have doubtless in 
large part been evolutionarily acquired in conjunction with the 
corresponding features of the flowers. Insects, however, are not 
the only animals thus utilized, for certain nectarivorous birds, of 
which the Humming-bird is the most familiar example, cross 
pollinate flowers in quite the same manner as the insects. Every- 
body has seen in our own gardens the Trumpet-creepers and 
Nasturtiums and Scarlet Salvias visited by Humming-birds. 
There are plenty of tropical flowers, displaying for the most part 
large tubular corollas, abundant nectar, and scarlet colors, which 
have a form, size, and shape well suited to the flying-habits of 
those birds. Among other animals that effect cross pollination 



324 The Living Plant 

arc the Snails, which are said to visit some low-growing flower- 
spikes of tropical plants for the soft tissue that grows abundantly 
among the blossoms; and thus they transfer pollen from one 
plant to another. But the other groups of animals are unavailable, 
for obvious reasons of habit, size, structure and the like. 

As an earlier chapter (on Protection) has indicated already, 
plants are obliged not only to develop structures in adaptation to 
the performance of their functions, but also to protect them when 
made from hostile external forces which would work their destruc- 
tion. This is all very true of the highly complicated and greatly 
exposed flowers. A certain protection against hostile weather 
conditions is attained by a control over the time of blossoming, 
which occurs in most plants only at times and seasons when the 
conditions are favorable for cross pollination, the blossoms open- 
ing in fine weather when insects are about, but not during rain- 
storms, w r hen they remain under shelter. One of the greatest 
dangers to which the cross-pollinating mechanisms are liable is 
the influence of rain on the pollen, for water is absorbed os- 
motically by many kinds to a degree winch causes the bursting and 
destruction of the grains. Accordingly, in flowers many arrange- 
ments exist in adaptation to protection of pollen from rain, aside 
from the great one already mentioned, the failure of blossoms to 
open in stormy weather. Thus, in a great many blossoms the 
anthers are safely sheltered under an overhanging upper lip, as in 
most irregular flowers, like the Mints, Monkshood, and others of 
horizontal position, while in some kinds they are guarded by 
bands of unw r ettable hairs. Again, some kinds of flowers close in 
threatening weather, while others, arranged in flat-topped clusters, 
turn upside down in a rain-storm, presenting an aspect wiiich 
leads most people to imagine that they have been beaten over by 
force of the rainfall. 

But an especial protective need of flowers is against insects that 
are not adapted to cross pollinate them, and which would remove 
the nectar without rendering any service in return, against 




Arrangements for Securing Union of Sexes 325 

"unbidden guests/' as Kerner so happily called them. In one 
instance, at least, plants seem quite helpless against such an 
attack, for Bees often puncture the nectariferous spurs of Colum- 
bines and Larkspurs in our gardens without entering the flower at 
all; but this is exceptional, and , > : 

presumably a recently-acquired 
habit of those insects. A partial ; 
protection against unbidden guests 
is secured by the adaptation of 
floral to insect shapes already de- 
scribed, in correlation with which 
most insects visit only the flowers 
to which they are fitted, leaving 
the others alone. But there is one 

IdnH of irmppf wVin^P <*mflll <5i7P Fio. 119. Interior of a flower of Cotow 
Kind 01 insect, \\nose bmall Size 8canden8% showing the masses of hairs 

and other characteristics make it commonly believed to protect the 

neetar from insects unadapted to ef- 

USeleSS as a CrOSS pollinator, but feet cross pollination. (Copied from 
.... ... . . Kerner's PJlanzenleben.) 

which is at the same time a par- 
ticularly pertinacious nectar lover, and that is the Ant, against 
which, accordingly, especial protection is needed. A number 
of adaptions preventive of its access to nectar appear to 
exist. Possibly the extra-floral nectaries earlier described 
(page 212), may provide a bait to keep these insects from 
the flowers. Furthermore this is probably the explanation of 
the closure of the throats of flowers, best exemplified in the 
Snapdragon, in a way to open by the pressure of a large insect's 
weight or strength but not to the small body of an ant; while the 
rings of scales or hairs in the throat or somewhere in the tube of 
the flower (figure 119), or sticky glands all over the outside of the 
calyx or neighboring parts (figure 120), have probably the same 
explanation, as have a number of other arrangements of minor 
account described by Kerner in his charming book devoted to the 
subject. 
It is thus plain that flowers, like other parts of the plant, are 




326 The Living Plant 

never the expression of adaptation to some single function alone, 
but represent a resultant or compromise between adaptation to 
some leading function and adaptation to a number of minor ones, 
the whole being further modified by the influence of a quantity 
of other factors, mechanical, incidental and 
hereditary. 

In this discussion of cross pollination and the 
flower, which involves some of the most com- 
plicated and efficient of all known adapta- 
tions, the reader must have noticed how closely 
the mode of presentation of ideas, and even the 

Fio. 120. A flower of . , . , , . , , 

theLwmaa f orTwin- language that is used, correspond with those 
adh^gS t which are commonly employed in describing 
the base of the flower, some great product of human activity, the 

supposed to protect 

it from access of organization of society, government, or a great 
creeping in^ecs. b us i ness . And this peculiarity of exposition 

is not confined to the present writer alone, but seems una- 
voidable by any author who seeks to make the subject under- 
stood. It arises of course in some part from our common custom 
of personifying nature for purposes of convenient, economical, 
and vivid expression, but in much larger part, I am convinced, 
from a more or less unconscious recognition of the fact that there 
is an actual correspondence, or even an identity, between man's 
way of effecting results, and nature's. It is not that nature thinks 
things out as a man does, but that mind in a man works things out 
as nature does. This must be true, indeed, on theoretical grounds, 
else we must maintain that the mind of man is not an evolution 
with its roots in the rest of nature, but a special creation of its 
own separate kind; and against such a conception is arrayed all 
of the natural knowledge we possess. In all exposition, therefore, 
it is, as I think, scientifically correct as well as practically con- 
venient, to personify nature. 




CHAPTER XIII 

THE WAYS IN WHICH PLANTS INCREASE IN SIZE AND 
FORM THEIR VARIOUS PARTS 

Growth; physiological 

j|F all the physiological processes of plants, the one that 
possesses the greatest interest for most people is 
Growth. It is really a remarkable phenomenon, no 
matter how one views it, whether in the unfolding 
and perfecting of some favorite flower, foliage, or fruit: in the 
development of a single microscopical egg-cell through embryo 
seedling and sapling to a mammoth tree : or in the seasonal proces- 
sion of vegetation from the dormance of winter through the un- 
folding of spring, the maturity of summer, and the fruition of 
autumn. I take it the reader does not share in the mischievous 
fallacy that to know the causes of things is to lessen one's enjoy- 
ment of them, and I shall try to describe the way in which these 
various results come about. 

At first sight the phenomena of growth seem too heterogeneous 
for analysis, but, like many another complication, they separate 
out in their true proportions under persistent investigation. And 
the first far-reaching fact which stands out is this, that growth 
consists of three operations, which, often in progress together, are 
really distinct in their nature and can proceed quite apart from 
one another. These are, formation of new parts, or development, 
increase in size, or enlargement, and ripening for the final func- 
tion, or maturation. The distinction comes out very well in the 
case of the spring vegetation. Everybody knows that the flowers 

327 



328 The Living Plant 

and leaves which burst forth at the first coming of spring were 
formed, or developed, the season before, and existed over winter 
tucked away very snugly in well-covered buds. In a Horse 
Chestnut bud, for example, one can recognize by dissection, at 
any time in winter, the flowers and leaves which are to come out 
the next spring; and the same thing can be seen even more clearly 
in sections made through flowering bulbs (Hyacinth, Tulip, 
Crocus) . Seeds with their embryos act the same way. In all of 
these cases the formation or development of the parts takes place 
in early fall; the principal part of their increase in size, or actual 
growth, occurs the next spring; while the full ripening of parts, 
such as leaves, for the complete performance of their functions, 
follows in summer. This shows how distinct the three phases of 
growth can be. Accordingly we can best consider them separately, 
and for practical reasons may begin with the most familiar, 
increase in size, or enlargement. 

Plants, unlike animals, grow by repetition of similar parts, 
new leaves, stems, roots, flowers, and fruits being formed in an 
endless succession. We shall therefore first direct our attention 
to the growth of these individual parts, of which the stems grow 
the fastest and are easiest to study. Anyone can determine the 
rate of growth of stems in a general way by making frequent 
measurement with rulers placed alongside the plant. For scien- 
tific purposes, of course, very exact ways have been devised, not 
only for measuring growth, but even for compelling a growing 
stem to register its own growth upon paper. One of the best of 
such instruments is shown in our accompanying figure (figure 121), 
and the reader may confide in my judgment of its merits, because 
I am myself the inventor. To the extreme tip of the stem is 
attached a thread, which is then run over a small wheel, as shown 
in the figure, and there fastened. Around the rim of the larger 
wheel, which is one piece with the smaller, runs another thread 
which passes over a small pulley-wheel and carries a pen against 
a paper-covered cylinder. The weight of this pen just suffices to 



Ways in Which Plants Increase in Size 



329 



turn the wheels and keep the threads taut; and therefore, as the 
plant grows, the pen descends, making its mark upon the paper. 
The descent of the pen, however, is obviously faster than the 
growth of the plant in just the proportion that the greater wheel is 
larger than the smaller, this arrangement of the wheels being 




FIG. 121. An auxograph, or recording growth measurer, in action. Its construction is ex- 
plained in the text. Unfortunately the record, in the form of a spiral line on the 
cylinder, does not show in the picture. 

adopted in order to space out the growth record enough for clear 
visibility. The cylinder, however, is revolved continuously by 
clockwork, making a complete turn once every hour; and there- 
fore the descending pen traces not a straight, but a spiral, line, 
which every hour crosses a vertical line ruled on the paper, mark- 



330 The Living Plant 

ing off thereon the precise growth, magnified of 
course proportionally throughout. The papers can 
then be removed from the cylinders and joined end 
to end in a continuous roll, or else a flat band. Thus 
is a plant made to write its own record of growth in 
a way convenient for scientific use. Such a record, 
obtained by one of my own students, and showing the 
growth of the flower-stalk of a Grape Hyacinth from 
its first appearance above ground to the completion 
of flowering, is shown, greatly reduced of course, in 
the accompanying illustration (figure 122). And 
by suitable modifications of the same auxograph (for 
so it is called because it is a growth writer), the 
growth of roots, leaves and other parts can likewise 
be registered. 

A growth record like that of our figure is very ex- 
pressive, but the facts can be brought out still better 
in the form of a graph like that which already has 
been used and described under Transpiration; and 
such a graph is presented in our figure 123. The 
base line is laid off in divisions of time, each space 
representing one hour, while the vertical lines are 
marked off with the number of millimeters of growth 
(magnified) per one-hour period, these marks being 
joined by straight lines in the usual way. In the 
resulting polygon, as the reader can see, the rise and 
fall of the lines corresponds to the rise and fall in the 
rate of growth. The reader must remember that such 
a graph represents the rate of the growth, not its 
amount, which fact explains the feature, puzzling 
to some people, that a growth graph can fall as well 
FIG. 122. Pho- as rise. 

tograph, re- 
duced to one-tenth the true size, of the record papers taken from the cylinder of the 
auxograph (of figure 121) during the growth of a flower-stalk of Grape Hyacinth. 
The heavier cross lines indicate noon of each day. 



332 The Living Plant 

When, now, we inspect this graph somewhat closely we find 
its most remarkable feature to consist in its great irregularities; 
and the same thing appears in any others, from whatsoever source 
they are taken. In other words, the growth of plant-structures is 
extremely irregular in rate. It will not take the reader very long 
to ascribe the irregularities to the real cause of the most of them, 
namely, variations in the external conditions of temperature, 
light, moisture and so forth. In order to determine the precise 
effect of each of these conditions, it is only necessary to plot the 
simultaneous graphs of temperature, moisture, and light, ob- 
tained as already described under Transpiration, upon the same 
sheet with the growth graph; and this has been done in the ex- 
ample presented above (figure 123). This subject of the effect of 
external conditions upon growth is, however, so important, that it 
must be considered somewhat farther. 

First, as to the effects of temperature upon growth. Every- 
body knows, in a general way, that plants grow faster in warm 
weather and slower in cold; and in the early spring we see ample 
illustration thereof in the way the grass comes up fastest in the 
warmest corners, or in places where warm pipes, such as sewers 
from houses, cross lawns, marking their courses by the early 
greenness above them. In our graphs the reader can see how 
closely the rise and fall in growth rate is connected with the rise 
and fall of the temperature. The same thing is shown, and very 
much clearer, by an instrument, devised for the purpose, and 
shown in our figure (figure 124). It must suffice to say that by 
its aid one can determine in a continuous band of soil the lowest 
temperature at which a plant can be made to grow (the minimum}, 
the temperature at which it grows its very best (the optimum), 
and that above which it will not grow at all (the maximum). 
Between the minimum and maximum, the tips of the growing 
plants plot, as it were, their own curve of the relation of growth 
to temperature, culminating at the optimum, as our picture well 
shows. 



Ways in Which Plants Increase in Size 333 

These three cardinal points vary much with different plants, 
ranging lower in those of cold regions and higher in those of 
the tropics; and plants can thrive only in climates where the 
range of usual temperature corresponds somewhat closely with 
their cardinal points. This will explain why the Orange will not 
grow if planted in Canada, or Barley and Rye if taken to Florida. 
In plants of our own climates these points approximate on 
the average to 5-30-40 Centigrade respectively (or 40-85- 
100 Fahrenheit), which means that most of our plants do not 
grow appreciably below 40; they grow best at about 85; and 




FIG. 124. A graphic illustration of the relation of growth to temperature. The copper 
trough is heated from one end (the left), and chilled from the other, with the result 
that the temperatures grade evenly between. 

hardly grow at all above 100. This will explain why it is that 
when the temperature of our fields rises higher than 100 in the 
sun, the extra heat is no aid to plant growth, being rather a 
hindrance thereto. The same thing would happen also in green- 
houses in summer were it not for the shading, which is added to 
reflect both the heat and the light. 

The reason why heat has this effect upon growth is fairly well 
known. Growth depends upon a number of chemical and physical 
processes which are kept in orderly cooperation by the protoplasm. 
All of these processes, in general, are promoted by higher tem- 
perature, which fact explains the more rapid growth up to the 
optimum point; but, as the temperature rises higher, to degrees 
beyond those to which the plant is accustomed, the processes get 
beyond control of the protoplasm, or run away, so to speak, thus 
injuring and finally destroying the coordination and stopping the 



334 The Living Plant 

growth. Other things, also, contribute to the result without 
doubt, such as the commencement of injurious chemical reactions 
under the higher temperature, and the accumulation of the waste 
products which are formed faster than they can be removed. 
But in general the relations existing between temperature and 
growth are determined by the power of the plant to control the 
chemical and physical processes concerned. 

Second, as to the effects of light upon growth. At first thought 
one would suppose that plants must grow best in bright light, 
since light is essential to the making of their food, which supplies 
both the material and the energy for their growth; but in truth 
it is usually more rapid in darkness. This fact is brought out in 
our graph (figure 123), though here, as is usually the case, the 
matter is much complicated because the temperature commonly 
falls so greatly at night as to neutralize any tendency the plant 
may possess to grow faster at that time. But when the tempera- 
ture remains even, as happens at times on warm nights out of 
doors, and in greenhouses artificially heated, then most plants 
show a tendency to grow faster in darkness. These are the con- 
ditions under which the farmer comments on the great growth 
that his cucumbers, for example, made in the preceding night. 
Plants make ample food in the day to supply the growth through 
the night. When, however, plants are kept continually in the 
darkness for days together, their growth becomes spindling and 
weak, and their chlorophyll disappears, as our picture will illus- 
trate (figure 125). The results of such growth are comparable, 
in general, with the weakening activity of a fever. 

The reasons why plants grow best in the dark are several. A 
part of this growth consists in that adaptive lengthening (the 
11 drawing" of gardeners) already considered in our third chapter, 
whereby plants reach up after light. It is well illustrated by the 
great length of the stems in our picture (figure 125). A part may 
result from the fact that during the day all other processes are 
subordinated to photosynthesis, while at night growth has the 



Ways in Which Plants Increase in Size 



335 



field to itself. A part depends on direct injury done by bright 
light through the injurious chemical reactions set up in the com- 
plicated protoplasm, a matter we have considered pretty fully 
under Protection. On green plants, of course, the action of light 
is far less injurious than on colorless kinds, because the chloro- 
phyll incidentally forms an excellent protective screen. In chief 
part, however, the lesser growth of plants in light is due to the 




FIG. 125. Pots of Scilla, started alike; but that on the right was kept in a dark room. 

great promotion of transpiration by the light and its associated 
heat, whereby so much water is removed from the plant as to lessen 
the supply available for swelling the growing cells, for such swell- 
ing is essential to their growth, as will be noted more fully a few 
pages later. 

Thus, it is plain that light, like heat, can become too strong for 
the best growth of plants. We have seen already that even in 
photosynthesis plants cannot make use of all the light supplied by 
direct bright sunlight. These facts together explain why so many 



336 The Living Plant 

plants thrive better in some shade than in full sun; and it is inter- 
esting to note that man finds it best to temper the light for some 
of his crops. This is the reason why shading is placed upon green- 
houses in summer, and why better tobacco is grown under light 
cotton tents than in full sun, though here the protection given by 
the tents against hail storms and wind is also important. In 
Florida, pineapples grow better under a lattice work shade than 
in full open sun. 

Third, as to the effects of humidity upon growth. A full supply 
of water in the soil is essential to the process, for this is the source 
of the water used in swelling the small new cells to their full adult 
size. But in addition the amount of moisture in the air has an 
important influence. Most people know that plants grow best 
on the kind of day we call "muggy," i. e., one in which the air 
is humid, even to the point of discomfort for us; and it is a familiar 
experience that upon such a day the grass of a lawn fairly grows 
before the eyes. The influence of humidity in promoting growth 
can also be traced in our graph (figure 123), which shows that in 
general growth increases with atmospheric humidity. The chief 
reason for this relation is easily found. Increased humidity 
checks transpiration, and therefore leaves in the plant a larger 
water supply for use in swelling the growing cells. 

Fourth, as to other influences which affect growth. These are 
few and comparatively unimportant. Electricity, applied ex- 
perimentally in limited amount, stimulates growth to a certain 
extent but in larger amount checks it; but its influence is not 
wholly separable from that of heat, and the matter is not so very 
important, since plants are hardly at all exposed to it in Nature. 
Poisonous substances in soil or atmosphere often stimulate growth 
a little at first, though ultimately they check it, through the in- 
jury they do to the living protoplasm. The presence of a little 
ether in the air seems, however, to promote growth without sub- 
sequent detriment, though the reason for this effect is not under- 
stood. The varying pressure of the atmosphere, recorded by the 



Ways in Which Plants Increase in Size 337 

barometer, should theoretically have some slight effect, but 
hardly any is appreciable in practice. 

If now, we return to the graph of growth (figure 123) and pro- 
ceed to eliminate those fluctuations which are traceable to tem- 
perature, light, and moisture, there still remains one peculiarity 
of much consequence, viz., a gradual rise in the graph as a whole, 
followed by a more abrupt descent. This means that the flower- 
stalk of the Grape Hyacinth, even when all disturbing external 
factors are eliminated, does not by any means grow at a uniform 
rate from start to finish, as one might naturally suppose, but, after 
beginning, grows faster and faster up to a point of highest rate, 
beyond which its growth is slower and slower until it stops. This 
peculiarity of growth, however, is not confined to the flower-stalk 
of this plant, but is very wide spread; and it has even a name of 
its own, viz., the " grand period." Thus, it is characteristic of 
winter buds; and this explains a phenomenon in connection with 
their opening which most people must have noticed, viz., that 
buds swell very slowly at first in the spring, seeming to take an 
interminable time before they show their green leaves, after 
which they open out very quickly, almost over night as it seems, 
to nearly the full size of their parts; and then they complete their 
final growth rather slowly. This opening takes place on the crest 
of the grand period as a rule, although it is complicated of course 
more or less by the effects of temperature. Leaves, single flowers, 
germinating embryos, fruits, and a good many other parts display 
the grand period. It is not, however, universal; for some struc- 
tures, like stems which continue their growth all summer, pursue 
an even course affected only by varying temperature, moisture, 
or light. 

By suitable modifications in details, records may also be se- 
cured by the auxograph for the growth of leaves and of roots. 
The graphs in general are very similar to those obtained from 
stems. But there is one feature of the growth of leaves, stems and 
roots, about which the auxograph gives no information, namely, 



338 



The Living Plant 




the place of most active growth in each part, whether at tip, base, 
or all through the structure. This, however, is easily determined 
in another way, viz., by marking the parts when young by evenly- 
spaced lines, the spread of which, as the parts grow up, must 

reveal the place where these 
grow the most. If a young 
root be thus marked by cross 
lines, the result is like that of 
our figure (figure 126). Evi- 
dently young roots grow almost 
wholly at their tips. If stems 
be marked in the same way, the 
result is somewhat different 
(figure 127). Evidently young 
stems grow mostly at their 
tips, but over a much larger 
area than the roots, as indeed 
one might infer from the way in 
which the nodes of young stems 
spread apart. It is no trouble at all to find an adaptive reason for 
the difference in the mode of growth of roots and stems, when one 
recalls that roots must pick their way through the irregular in- 
terstices of a closely-pressing soil, while stems have all outdoors 
to expand in. As to leaves, their shape makes it necessary to 
mark them by cross lines, forming squares, and when thus treated 
the spread of the lines shows that leaves, unlike roots and stems, 
grow all through their structure (figure 128). Slender leaves, 
however, especially the kind that grow up from bulbs, grow al- 
most wholly at the base. 

Although growth is typically accompanied by increase in 
length, it sometimes is correlated with shortening. One case 
thereof is found where a straight structure becomes a spiral, 
as in tendrils, which thus pull their plants closer to a support, 
or in the peduncles of some water plants, which thus draw their 



Fir,. 120. A young Bean root, 
just marked by evenly spaced 
cross marks, and the same root 
a day later. 




Ways in Which Plants Increase in Size 339 



ripening fruits to a safer position under water. But an actual 
shortening occurs in the roots of some herbaceous perennials, 
like the Dandelion, which thus are enabled to keep their stems 
safely underground despite a certain annual increase in length. 




A 





FIG. 127. A stem 
of Melothria, just 
marked by 
evenly- spaced 
cross marks, and 
the same stem a 
day or two later. 




The same thing is said to occur in the lateral rootlets of some 
bulb-bearing plants, like the Tulips, with this marked advantage, 
that the newly formed bulblets are drawn clear of the old parent 
bulb. Mechanically, this shortening is variously effected, but 
chiefly by a forcible lateral expansion of the tissues, somewhat on 
the principle by which a muscle is shortened; and as a result 
such roots commonly show a number of transverse wrinkles. 



340 The Living Plant 

Such are the principal phenomena of that phase of growth 
which is concerned with enlargement. Another phase is con- 
cerned with the formation of new parts, or development. But 
the relations of the two will be much plainer if, before proceeding 
with the latter, I describe the cellular basis of both. As to this, 
we may anticipate a little by saying that in general, enlargement 




Fio. 128. A young leaf of English Ivy marked in regular squares, and the same leaf a 

week or two later. 

depends upon swelling of cells already formed, while develop- 
ment, or the construction of new parts, rests upon the formation 
of new cells. 

The mode of formation of new cells is singularly uniform 
throughout all plants. It takes place, as a rule, only in small 
compact thin-walled cells densely filled with protoplasm, the 
kind technically known as meristem and best shown at the growing 
points of stems and roots (figures 53, 137, 139, C. D). The details 
cannot be seen in living cells, but can be inferred from the appear- 
ances presented by cells killed, stained, and sectioned for the pur- 
pose. The first sign of new cell formation occurs in the nucleus 
(figure 101), where the granules become more conspicuous and col- 
lect into stout threads which then sort themselves out in the form 



Ways in Which Plants Increase in Size 341 

of a definite number of the bodies called chromosomes; and these 
become arranged in a plate across the cell. Meanwhile the bound- 
ary of the nucleus has vanished, and a spindle-shaped framework 
of very fine fibers has formed at right angles to the chromosome 
mass. Then each chromosome splits lengthwise into two, and the 
spindle draws these halves apart towards its two ends, where 
they become surrounded anew by a nuclear boundary. Thus is 
the chromosome matter divided evenly between the two new 
nuclei. The chromosomes then lose their distinctness and grad- 
ually merge away to the threads, and finally to a granulation 
similar to that of the original nucleus. Meantime the spindle 
fades away and a new wall forms across the cell between the new 
nuclei. Each of the new cells then grows to the original size and 
is ready for another division. 

The object of this elaborate process is without doubt the equal 
division of the chromosomes. These, it will be remembered, are 
derived equally from the two parents of the plant, half of them 
from one parent and half from the other; and although they ab- 
sorb nourishment and grow and divide, they never lose their 
identity. The equal division of the chromosomes in every division 
of the cells, therefore, carries some of the substance derived from 
each parent to every cell of the adult plant, thus explaining how 
it is that any part of a plant can resemble either one of its parents. 

Cell division underlies all development of new parts, for every 
structure leaf, stem, root, or other begins with the formation 
of just so many cells at just such places as will produce, when they 
swell to full size, the characteristic size and shape of the fully-adult 
organ. But at first these cells are all small, and densely packed 
with protoplasm and food substance. Such is the condition in a 
bud or an embryo, as our figures illustrate (figures 137, 139, C). 
One must not, however, lay too much stress upon the cell divisions 
in particular, for they are without doubt a result, rather than a 
cause, of the outgrowth of new parts. It is in reality the living 
protoplasm which pushes out into new structures; the cell divi- 



342 The Living Plant 

sions take place as a secondary architectural arrangement. It is 
easy to follow the method whereby the individual cells grow from 
the tiny food-packed condition to the large protoplasm-lined and 
water-filled state that distinguishes them when adult; and the 
matter is well illustrated in the accompanying figure 129. First 
of all, inside the dense protoplasm there appear little rifts which 
contain a sugar-rich sap. Into these little sap-cavities water is 
absorbed osmotically, making them swell and exert pressure 
which pushes the protoplasm against the walls and stretches them 
tensely. But this pressure is relieved by the deposition of new 
substance all through the innermost texture of the stretched wall ; 
and this allows a still further stretching, and so on until the cell is 






FIG. 129. Generalized drawings, in optical section, of a cell during enlargement from the 
newly developed to the fully-adult condition. 

full grown. The sap-cavities, meanwhile, are not only enlarging 
but are merging together; and the food substance originally 
stored in the cell is being transformed into new cell-wall, proto- 
plasm, and materials dissolved in the sap. The final product is a 
fully-grown cell, many times larger than its embryonic original 
and provided with a tightly-stretched wall against which lies 
a thin lining of protoplasm, enclosing a single sap-cavity well- 
nigh as big as the cell itself. The exact direction of expansion of 
the cell, and therefore its final shape, are of course by no means 
accidental, but are under control of the living protoplasm, which 
thus simply makes use of osmotic pressure as the mechanical 
power for forcing cell enlargement. And the degree to which 
that enlargement may proceed, from the newly-developed to the 



Ways in Which Plants Increase in Size 



343 



fully-adult cell, is sometimes surprisingly great, as the accom- 
panying example well illustrates (figure 130). 

From these considerations it will be plain that the fully-adult 
cell consists largely of water, with comparatively little solid 
matter, in great contrast to the embryonic cell which is largely 
solid. This is shown very clearly by the great 
collapse of fresh plant-structures when dried \\ 

(for often they shrink away to a mere wisp of 
their former selves), and also by weighings, 
which prove that most fresh plant-structures 
consist of more than 90 per cent water. A 
plant as large as that shown in our figure, for 
example, (figure 131), can be contained when 
dried in the tiny vial beside it. The same thing 
is true also of seedlings and the spring vegeta- 
tion from buds; when the water is expelled, it 
is found that the fully grown structure is not 
only no heavier than the embryo or bud, but 
even lighter in weight, the loss of course be- 
ing due to the removal of material by respira- FIG. 130. The 
tion. Thus in general it is true that developing 
structures gain weight, while growing struc- 
tures lose it. 

That growth consists chiefly in swelling of 
cells already laid down in development is 
shown very beautifully by comparison of some 
embryos with the seedlings that grow from them. If cross- 
sections of embryos and seedlings be made in about the same 
place, it is found on the average that although the cells differ very 
greatly in size, their number is approximately the same, though 
in one case they are tiny, squarish, densely packed and full of 
substance, while in the other they are large, rounded, loosely- 
arranged, and contain little but water. This separation of devel- 
opment and growth is more common than one would suppose, for 



parative sizes of n 
pith cell in the newly 
developed and tho 
fully-adult condition, 
as seen in optical 
section. Traced from 
accurate drawings on 
a wall-chart by Frank 
and Tschirch. 



344 



The Living Plant 



even in structures which grow on continuously, and in which it 
would seem that the two phases must be mixed up together, they 
are separated in space, although not in time. Thus, in roots, the 
development of new cells occurs in the growing point (figure 53, 
139, D), while the enlargement of cells to full size takes place in the 




FIG. 131. A Castor Bean plant, with its dry substance in a vial alongside. (The vial, of 
course, was photographed later, and worked into the plate.) 



zone just behind, a fact which explains the enlargement of that 
zone as shown in our earlier figures. The same is true likewise of 
the stem, though less strikingly. Moreover, it is also a very inter- 
esting fact that if a plant is suddenly called upon to increase be- 
yond the normal, as for example in the longer leaf -stalks demanded 
of water plants forced to grow in deeper water, or in the leaves of 
plants whose buds have all been destroyed, the enlargement is 



Ways in Which Plants Increase in Size 345 

attained by increasing the size of cells beyond the normal, not by 
increasing their number. 

But while enlargement and development are separate in their 
nature, and commonly occur apart from one another, neverthe- 
less they are often intermingled more or less. The very act of 
development, indeed, entails some increase of size, and enlarge- 
ment is attended by some cell divisions in connection with adjust- 
ment of parts; and no doubt, furthermore, there are structures 
which develop and enlarge simultaneously. 

As growth comes near to completion, and sometimes much 
earlier, the cells undergo such further changes as fit them more 
perfectly for particular functions. Such changes are designated 
maturation. Walls thicken in places and are absorbed in others; 
they develop spirals, rings, or other thickenings, and hollow pits 
or other depressions; while various changes take place as well in 
the contents, which often are transformed to secretions of very 
specialized function. Moreover, as cells increase in perfection of 
adaptation to their functions, they lose at the same time their 
power of division, so that when fully mature they are incapable 
of further development or reproduction. But these changes in 
the main have already been considered in the chapters on Pro- 
toplasm and Metabolism. 

Before leaving this aspect of growth, we should summarize for 
completeness the other physical and chemical phenomena thereof, 
most of which have been considered in various connections earlier 
in this book. Thus, there is always a large conversion of stored 
food into new walls, protoplasm, and sap substances, resulting in 
the collapse of the storage parts of sprouting structures, like 
potatoes, bulbs and seeds. Again, respiration, the releaser of 
energy, is indispensable to growth, which demands much of it; 
and so close is the connection of the two, that whatsoever stops 
the one stops also the other. Therefore, oxygen being essential to 
respiration, if the oxygen supply be cut off from the growing 
plant, as happens often in nature through flooding with water, 



346 



The Living Plant 



and as can easily be effected by experiment, then growth ceases; 

and indeed death ensues unless the supply be admitted again. 

Furthermore, in the chemical reactions of growth, some waste 

by-products are formed, of which a part are dropped with the 

bark and the leaves, a part 
are stored in out of the way 
cells, and a part are appar- 
ently excreted into the soil, 
where they act poisonously, 
and produce economic and 
ecological consequences al- 
ready described. 

A notable feature of growth 
is its accompaniment by a 
number of different move- 
ments. Many of these are 
clearly adjustive of the parts 

to the particular conditions 

of light, moisture, and so forth 
prevailing in the immediate 

FIG. 132. An arrangement (about one-sixth environment, and RS SUCh 

S^^^dSSw rtTSS: have been considered already 

Thorium filament and paper triangle, some- j n Qur chapter Oil Irritability, 

what exaggerated for visibility in the draw- * u 7 

ing, may be seen near the center of the while they will also receive 

further mention in suitable 

places in the chapter that follows. There is, however, one move- 
ment of which the description belongs here, since it is an inciden- 
tal accompaniment of all growth. It is that which was named by 
Darwin, its discoverer, circumnutation. So slow is it, ordinarily, 
however, that special methods are needed to render it apparent. 
If one takes some young seedling, such as Radish or Corn, attaches 
alongside its tip by harmless cement a slender projecting glass fila- 
ment, places black reference marks on the end of the filament and 
on a bit of white paper at its base, and then supports a pane of 




Ways in Which Plants Increase in Size 347 

glass horizontally a foot above it (figure 132), he can, by sighting 
his reference marks, record on the pane the spot to which the fila- 
ment is then pointing. But if, a half hour later, he sights again, 
he finds that the filament, and therefore the tip of the plant, 
points in another direction, and later in another, and so on. By 
drawing straight lines through the points thus established, one 
obtains a kind of polygon representative crudely of the magnified 
course of the moving tip of the seedling; and a few of these records, 
traced by one of my own students, are given herewith (figure 133), 
while Darwin's book, The Power of Movement in Plants, contains 
a great number. These are not by any means isolated cases, for 
comparative studies have shown that such movements are dis- 
tinctive of most if not all growing parts, stems, buds, leaves, 
roots, tendrils, flowers and their parts, and many others, all of 
which, move during growth in slow, irregular, and jerky paths, 
that are longer and more rapid the more active the growth of the 
part. While the movement is thus well-nigh universal, it is not 
popularly known because of its slowness. If its rate could be 
magnified a few dozens of times, what a different aspect would 
vegetation present ! Then all the visible parts of all the growing 
plants of a garden, a meadow, or a forest, would exhibit a con- 
stant irregular movement, which collectively would seem of a 
tremulous character, much, I imagine, as would be shown if 
the plants were shaken by continuous little earthquakes. 

As to the cause of the circumnutation, that is known, in prin- 
ciple at least. It results from the fact that all growing structures, 
utilizing as they do osmotic turgescence for the expansion of their 
tissues, are under strong internal pressures which hold them in a 
highly tense but unstable stiffness. Now the readjustment of 
these pressures in growth cannot proceed with perfect evenness all 
around the stems or other parts, whose great length and slender- 
ness cause a large magnification of even the slightest disturb- 
ances of the equilibrating tensions, and circumnutation results. 
These movements, therefore, are simply an incidental by-product 



The Living Plant 



'*. 




FIG. 133. Records of the circumnutation of some common plants, obtained by the method 
illustrated in figure 132. The letters h and m signify hours and minutes between 
observations. 

of growth, and one of those incidental phenomena which possess 
no adaptational significance; and it is partly because it is so good 
an example of such incidental phenomena (of which autumn 
coloration, forms of starch grains, and phyllotaxy, are instances 



Ways in Which Plants Increase in Size 349 

earlier described in this book), that I give to it here so much atten- 
tion. There is, however, another reason for its consideration, 
namely, that Darwin considered it the starting point for most of 
the useful plant movements, the twining, sleep, geotropic, hydro- 
tropic and other adjustive movements which we considered under 
Irritability. His conclusion on this point, has not, however, been 
accepted by later investigators, though the present status of the 
matter may be expressed by saying that his view is unproven 
rather than disproven. 

Finally, as to the physiological phases of growth, there remains 
one matter which is both scientifically interesting and econom- 
ically important, and it concerns grafting. Everybody knows 
that small twigs of apples, cherries, pears and many other plants 
can be cut from those trees and inserted into the stems of others 
in such, way as to grow and form structurally an integral part of 
the new tree. Furthermore (and this is what gives to grafting its 
great economic importance), the inserted twig and everything 
which subsequently grows from it, continues to produce its own 
kind of leaves, flowers, and fruits substantially unaffected by the 
plant into which it was grafted; while, correlatively, the stock 
plant into which the graft was inserted continues to produce its 
own kind of vegetation unaffected by the graft, even though this 
may in time become the greater part of the tree. Thus it is 
possible to graft a number of very different varieties of apples, or 
of cherries, into a single trunk and produce a tree which bears 
all those varieties as long as it lives, without any visible sign to 
show that it was ever anything other than one tree from the start. 
It is in this way that highly specialized forms of fruits, leaves, or 
flowers, which appear mainly as sports (to be further considered 
in our chapter on Plant Breeding), and which cannot be grown 
from seed, are propagated and multiplied indefinitely. 

Turning now to the purely physiological side of grafting, the 
first fact of prominence is this, that the twig, which is called the 
scion, (or don), and the plant into which it is inserted, called the 



35 The Living Plant 

stock, must be closely related, else no union of tissues takes place. 
We find the same necessity in hybridization, or crossing of dif- 
ferent varieties of species by pollination; and indeed the possibili- 
ties of grafting and of hybridization have much the same limits, 
being comparatively easy between varieties of one species, much 
less so between species of the same genus, extremely rare between 
different genera, and unknown outside of the same family. Prob- 
ably the reason is a chemical one the more distantly related the 
forms the more likely is their protoplasm to contain chemicals 
which react on one another in a way to produce disturbing if not 
injurious or fatal compounds, thus preventing a normal or orderly 
continuance of growth. But when the protoplasm of scion and 
stock is actually congenial, so to speak, then the two grow to- 
gether precisely as a wound on one plant would heal up, and the 
tissues unite and thereafter grow as one single mass. It is neces- 
sary that a considerable area of living tissue be brought into con- 
tact, which is comparatively easy in these plants possessing a 
cambium cylinder (i. e. a continuous growth system soon to be 
described), but it is practically impossible in others. This fact 
explains why no grafting is possible among plants belonging to the 
groups of the Corn, Lilies, Palms, wherein no cambium exists. 

Although, in general, the scion and stock retain each its own 
characters unaffected by the other, a partial exception occurs in 
some minor features, such as earliness of blossoming, resistance to 
frost, and even some slight alterations in flavor of fruit or its 
color. In all these cases, I believe, such characters can be traced 
to the influence of the sap, which of course moves from stock to 
scion, or of the food substance, which moves from scion to stock. 
The living protoplasm, however, does not thus move from one to 
another, but remains within the original cells, or those which 
grow from them; wherefore the characters which depend on the 
protoplasm, including substantially all of those which give the 
distinctive characteristics to plants, are never transferred from 
stock to scion, or vice versa. 



Ways in Which Plants Increase in Size 351 

From the facts just stated, it would seem impossible for graft 
hybrids, that is, intimate mixtures of the protoplasm of stock and 
scion, to exist. Yet graft hybrids have actually been claimed to 
occur, though very rarely. And here opens up one of the most 
interesting chapters in recent experimental studies, for it has been 
found possible to produce experimentally such apparent graft 
hybrids. But the very same experiments have shown that they 
are really not hybrids at all, but merely mixtures of the tissues of 
the scion and stock, and not a blending of their protoplasm. These 
experiments were made by grafting a part of a bud of the scion to 
a part of a bud of the stock, when the resultant branch displayed 
a most remarkable mixture of the colors, shapes, tissue characters, 
and other features of scion and stock not a blending but a 
mixture. Sometimes the upper side of the branch would be all 
scion with the characters thereto appropriate, and the under side 
all stock; sometimes a sheath of stock enwrapped a core of scion; 
and other mixtures of other sort occurred. Such graft products 
are not hybrids, and have been named chimceras. But are graft 
hybrids then impossible? Theoretically they are not, for if one 
cut cell of the stock and one cut cell of the scion should happen to 
match together, and if then their two nuclei should fuse together 
(as they well might, for we know cases of fusion of nuclei other 
than in fertilization); and if from this hybrid cell there should 
then develop a branch by the ordinary process of cell division, 
then the cells of that branch would all possess protoplasm and 
chromosomes from both stock and scion, and a true graft hybrid 
would exist. This alluring possibility has naturally attracted 
the eager attention of the experimenters, and already they have 
announced success, though as yet of a somewhat unsatisfying 
character. And if by good fortune I have ever the privilege of 
preparing a new edition of this book, I shall probably be able to 
describe much that is important and interesting in this connec- 
tion; for this line of experimentation has opened up much more 
than merely this question of graft hybrids. 




CHAPTER XIV 

THE ORDERLY CYCLES PURSUED IN GROWTH, AND 
THE REMARKABLE RESULTS OF DISTURBANCE 
THEREOF 

Growth: structural 

||HE reader may possibly wonder, as he contemplates the 
chapter before him, what reason there is for its separa- 
tion from the one that precedes it, when both are con- 
cerned with the very same subject and closely inter- 
connected. So I may as well make the confession that it has not a 
much better basis than the reason assigned by an early French 
naturalist for excluding the Crocodiles from Insects, the animal 
seemed to belong there, but would make quite too terrible an 
insect! I like to conceive of this book as read one chapter at a 
sitting by a reader who has interest enough in the subject to make 
its careful perusal the chief feature of an evening's business; and 
so much must be said about growth that it cannot be followed 
unweariedly without some kind of division or intermission. 
However, the matter is really not quite so desperate as this, for 
the physiological and structural phenomena of growth are in fact 
sufficiently different to make a division between them not wholly 
unnatural. 

Of the structural phenomena of growth, the most striking and 
important are concerned with the cycle of development of the 
individual plant from its very first origin up to its adult condition; 
and this is comprised in four stages. 

1. The Growth Cycle; from Egg-cell to Embryo. This stage 
is rather well represented, albeit somewhat diagrammatically, by 

352 



The Orderly Cycles Pursued in Growth 353 



the accompanying picture (figure 134). The reader will recall 
that the egg-cell is the female reproductive cell formed inside the 
embryo-sac within the ovule, and that it needs to be fertilized by 
a male cell brought by a pollen-tube, before it can develop to an 
embryo. Immediately after fertilization, the egg-cell divides into 
two; these grow in size, and again divide, and so on in a way to 





FIG. 134. Typical stages in the development of an egg-cell into an embryo (of Rape). 
The original egg-cell lay at the bottom of the embryo sac of which a part is shown in 
the figure on the left, while the other figures show the development of the initial cell, 
at the top of the suspensor, into the embryo. (Adapted from pictures on a wall-chart 
by L. Kny.) 

produce a line of cells forming a structure called a suspensor, 
which carries a terminal, or initial, cell out into the middle of the 
embryo-sac, where there is ample space for the development of 
the forthcoming embryo. Then the initial cell begins to divide, 
first at right angles to the earlier divisions, then again and again 
in other planes with great regularity, as represented in our pic- 
tures, until finally a many-celled ball is produced. Then the 
regularity ceases, and cell division becomes more active at two 



354 



The Living Plant 



definite places, resulting in outgrowths which wax greater and 
greater until they become the thick leaves that later are called 
the cotyledons. Meanwhile the original ball is growing more 
actively at the opposite end, there producing a cylindrical struc- 

ture which forms the stem, or 
hypocotyl, of the embryo, while 
a group of growth cells at its 
tip forms the foundation for 
the forth-coming root, and 
f another between the cotyle- 

IIG. 135. Typical seeds, with embryos, of 

the two leading types; "albuminous" (a dons forms the foundation for 

Barberry) and "exalbuminous" (an Apple), ., n . . . * , m-i 

as further explained in the text. (Copied the nrst terminal DUCL ItlUS 
from Gray's Structural Botany). firgt 




and the foundations for the first root and bud, of the new 
plant laid down wholly inside the embryo-sac of the ovule, 
forming the structure which we call the embryo. Simultaneously 
the coats of the ovule are growing thicker and harder, the suspen- 
sor is being absorbed, and a supply of food substance, the endo- 
sperm, developed in a manner already described (page 299), is 
filling all the space in the embryo-sac not preoccupied by the 
embryo. The resultant structure, a combination of embryo, food 
substance, and protective coats, is the Seed (figure 135, on the 
left). 

Such is the typical method of development of embryos and 
seeds, though of course a great many differences occur in detail. 
In some seeds the development stops at the point here described, 
leaving the young embryo surrounded by copious endosperm or 
" albumen"; but in others, for example, Peas and Beans, the 
embryo continues its development until it has absorbed all the en- 
dosperm and everything else inside of the seed coats, in which case 
it usually develops also the first bud, called the plumule, between 
the cotyledons (figure 135, right). In any case, the seed is now 
ripe. It gives up most of its water, hardens its coats, separates 
from the parent plant, and goes into that resting state, in which 



The Orderly Cycles Pursued in Growth 355 

some kinds may remain, with vitality intact, for years and dec- 
ades, and even a century, though not for the ages implied in the 
current but groundless belief that genuine seeds from the wrap- 
pings of mummies will germinate. In this condition, small and 
light, and independent of external food or water supply, the seed 
is capable of wide transport; and thus forms a natural stage 
for dissemination, in adaptation to which its coats or neighboring 
structures often develop wings, plumes, hooks, pulp and colors, as 
we shall consider more fully in the following chapter. It is not of 
course because the seed has these characters that it is utilized by 
the plant as its dissemination stage, but it is rather because it has 
been developed as the dissemination stage that it has these char- 
acters. 

In following the sequence of cell divisions involved in these re- 
sults one cannot but wonder what the nature of the controlling 
power must be. Structurally considered, cell division can take 
place just as well in one direction as another, yet in fact it takes 
place in substantially the same directions as in preceding genera- 
tions of embryos, directions which bring an adaptive result. 
What is it which compels the developing egg-cell to form a line of 
cells instead of a ball, and the initial cell to form a ball instead of 
a line; which leads the ball to push out the two cotyledons in 
definite places, and to make the hypocotyl and root in another? 
In some way, it is certain, the control issues from the chromo- 
somes, which alone hold the knowledge of how the former genera- 
tions developed; but through what mechanism do they exert 
their authority? This question, for the most part, we cannot yet 
answer, but in some part we can; for it seems reasonably certain 
that most of the changes consist in responses to stimuli, the nature 
of which was explained in our chapter on Irritability. Perhaps 
the pressure of the egg-cell against the end of the embryo-sac is 
the stimulus which sends the suspensor developing as a single cell- 
line in the opposite direction; perhaps the freer osmotic absorption 
permitted by the arrival of the initial cell into the more fluid 



356 The Living Plant 

central part of the embryo-sac is the stimulus which sends this cell 
developing into a regular mul-ticellular ball; perhaps the beginning 
of pressure on this ball as its expansion brings it against the 
protoplasmic lining of the embryo-sac, is the stimulus which sets 
the cotyledons developing at their definite places, which places 
themselves may be fixed by the positions of least pressure; 
perhaps the contact of these growing cotyledons with one another 
is the stimulus which starts the development of the plumule be- 
tween them, and starts also the extension to form hypocotyl and 
root in the other direction. Maybe, or probably, I am wrong as 
to the details of these stimuli, but if it is not these it is some others 
of similar sort; and in any case my speculations illustrate the 
principle of the matter. The idea is confirmed by the fact that 
there is one case of growth stimulation of whose nature we are 
reasonably certain. The reader will recall that the stimulus given 
by the fusion of the second nucleus of the pollen-tube with the 
nucleus of the embryo-sac starts the development of the endo- 
sperm (page 299) ; and this, or some other phase of fertilization, 
is the stimulus which starts not only the hardening of the seed 
coats and the development of other typical seed features, but also 
the many large processes involved in the formation of the fruit. 
It is a fact that ordinarily neither endosperm, seed coats, nor fruit, 
develop unless fertilization is effected, an arrangement that is 
obviously adaptive, since without fertilization they would all of 
them be useless, and a wasteful drain on the plant. This kind of 
"linking up" of the processes together through the connection of 
stimuli is believed to be representative of the method whereby 
the development of plants, and animals too, is kept in harmonious 
and continuous progress. It is essentially the same method 
as that by which the parts of a complicated machine are kept 
working effectively together, each special part of the mechan- 
ism being geared or connected to some of the others in such man- 
ner that the movement of the mass as a whole compels each part to 
perform its destined office at just the right moment and place. 



The Orderly Cycles Pursued in Growth 357 

The analogy, indeed, goes a long way farther, for, just as the 
accidental loosening or breaking of some connection causes the 
machine to work irregularly, or even causes its different parts 
to work independently of one another, so the failure of the 
stimulus-connection in the organism may release some parts from 
the regulatory control of the remainder and cause them to work 
more or less independently. Such is without doubt the explana- 
tion of the abnormal or monstrous growths presently to be con- 
sidered. The same thing is well known in the animal kingdom, 
where tumors, for example, are known to be growths released in 
some way from the regulatory control usually exercised by their 
connection with the rest of the organism; and we have the same 
thing in mental phenomena, for dreams, in all probability, are 
simply mental processes whose correlation is temporarily lost in 
sleep, while insanity is the same thing with the correlation more 
or less completely or permanently lost. 

While the embryo is thus developing from the egg-cell and the 
seed from the ovule, the fruit is developing from the ovary and 
other parts of the flower; and this fruit aids in dissemination, by 
the methods we shall later consider. During dissemination, and 
often for long after, the seed remains in a resting state, with its vi- 
tality suspended. In most seeds this resting period is compulsory 
for a time, at least for several weeks, within which period the seed 
will not germinate no matter how favorable the conditions that 
may be offered. The same thing is true, by the way, of winter 
buds, bulbs, and some other plants, though it is interesting to note 
that many cultivated plants, notably the grains, have lost the rest- 
ing period, and will germinate as soon as ripe, even sometimes in 
the seed pod. The advantage of the resting period to the plant is 
sufficiently plain: it gives time for dissemination and it prevents 
premature germination, such as might happen during a warm 
time in winter, resulting in the destruction of the embryo by the 
subsequent frosts. It is effected and controlled, of course, by the 
protoplasm, which uses various arrangements for the purpose, 



358 



The Living Plant 



sometimes seed coats so constituted as to take days or weeks for 
water to penetrate them, sometimes a delay in the development 
of the enzymes needed to soften the endosperm, sometimes no 
doubt in yet other ways that are still undetermined. 

Thus is the new plant developed in the seed prior to its birth. 

2. The Growth Cycle; Germination. This is a very distinct 
though brief stage. When the resting period is completed, the 
seed germinates on the first access of water in conjunction with 
warmth. The water is absorbed and passed on to the embryo, 




FIG. 136. A generalized drawing of a typical case of germination, from the dry seed to 
the fully grown embryo. The controlling factors are discussed in the text. 

which swells powerfully, and thus bursts open the seed coats. 
Immediately the root grows rapidly out, and, no matter in what 
position the seed or embryo may happen to lie, invariably turns 
downward under the stimulus of gravitation, and, develop- 
ing a zone of anchoring and absorbing hairs, proceeds to grow 
straight into the earth (figure 136). This is an obvious adapta- 
tion to the new plant's first needs, a firm anchorage in the soil 
and a supply of water therefrom. When the root is thus firmly 
anchored, the embryonic stem, under the stimulus of gravitation, 
begins to turn upward, and, guided by other stimuli, works the 
cotyledons out of the seed, and carries them upward, where, 



The Orderly Cycles Pursued in Growth 359 

responding to the stimulus of light, they open out, turn green, 
and serve as the first foliage. At least such is the procedure in the 
most typical cases, though there are many variations in detail, 
including especially a great many cases in which the cotyledons 
remain in the seed, sending up only the plumule. Meanwhile 
this embryo has continued to absorb water, with which its cells 
have swelled greatly; and its stored food has been largely con- 
verted to new wall and living substance. Finally it stands up 
stiffly, many times larger than at first, but with no new parts and 
even less of solid substance. It is now all ready to begin its in- 
dependent life. 

Thus is the new plant born. 

3. The Growth Cycle: the Seedling. The stages of develop- 
ment, while distinct in the main, overlap in some places. Thus 
the root develops considerably in germination, and early in this 
stage begins to branch. Its new cells are all formed at a definite 
growing point, whence they radiate in regular lines backward, in- 
creasing in size, as shown very clearly in our earlier figure 53, and 
diagrammatically in a later figure, 139 D. The branches of roots 
start always from the fibrovascular bundles and have therefore 
to break or dissolve their way out through the cortex (figure 67), 
a method which seems clumsy, but is doubtless the best that the 
plant can do. Their places of origin are fixed largely by exter- 
nal stimuli, the contact of greater warmth, moisture, aeration, 
mineral supply and the like. The guiding stimulus of their sub- 
sequent growth is gravitation, which sends them radiately out- 
ward in directions of least interference with one another, though 
they, and especially the later branches, are swung from the 
geotropic angles, and given their final details of position, by ad- 
vantageous responses to various minor stimuli. Thereafter, so 
long as the plant lives, these roots grow, branch, and are guided 
continuously in this manner. Meantime the embryonic cells be- 
tween the cotyledons become active and push up a cone of cells 
which constitutes the first bud. As this bud becomes larger the 



The Living Plant 

cell divisions become more active at definite points near its base, 
and push out flat projections which develop and grow into the 
leaves, as shown by our accompanying figure 137, and diagram- 
matically by figure 139, C. Unlike new roots, the leaves have their 
places of origin determined not by stimuli from without, but by 
internal influences, for they come out from the bud in accordance 




Fio. 137. A bud (of Elodea, a water plant), in surface view and section, showing unusually 
clearly the mode of development of new leaves. (Copied from a wail diagram by 
L. Kny). 

with definite mathematical systems, as we have considered already 
under phyllotaxy (page 62). In their early stages, and while 
their tissues are still young, the leaves are flattened closely over 
one another into the conical structure we commonly call a bud; 
but as they become old enough to be useful they bend outward 
and ultimately present their inner faces to the sun. As they grow, 
their blades tend to take horizontal positions under guidance of 
gravitation, but they are easily swung therefrom, and given 



The Orderly Cycles Pursued in Growth 361 

final direction, by the stimulus of light, to which they set their 
blades at right angles. As the leaves develop, embryonic tissue 
in their axils becomes active, and develops into new buds pre- 
cisely like the first bud produced by the embryo, the stimulus 
thereto being probably the pressure ex- 
erted upon them by the developing leaf. 
It is easy to see an advantage in this 
axillary position of buds, for their first 
need is abundance of food, and the leaves 
are the source of supply. From these 
buds grow branches, the primary direc- 
tions of which are assumed under guid- 
ance of gravitation, whereby they are 
sent radiately out into positions of least 
interference with one another, although 
the details of their ultimate positions are 
fixed by a variety of minor influences, 
precisely as in the case of the root- 
branches above mentioned. 

Such is the complete structure of a 
seedling, of which a typical example is 
here represented (figure 138). 

4. The Growth Cycle: the Adult. The 
seedling continues development and FIG. 138. A typical seedling 

., p .j , , AT. (of a Maple), showing the 

growtn lor a considerable time in tne distinctive parts, excepting 
manner just described, branching con- ^k^ b " U (0 E 
tinuously into new roots and stems, and ied from Gra y' s structural 

^ ' Botany.) 

making new leaves. Its transition to 

the adult condition may be considered as marked by the beginning 
of reproduction, even though the plant may by no means have 
reached its full size. Suddenly, at some time in the plant's growth, 
without any apparent reason, some buds begin to produce flowers 
instead of more leaves. The central features of flowers are in 
reality the pollen grains and the embryo-sacs, and there can be 




362 The Living Plant 

little doubt that it is the beginning of the formation of these which 
gives the stimulus to the formation of sepals, petals, stamens, and 
pistils, instead of ordinary leaves. But it is not yet clear what it is 
which starts this formation of pollen grains and embryo-sacs, 
though it must result in part from some outside stimulus, since 
plants can be made to flower much sooner by making the external 
conditions somewhat harder. The flower, once formed, secures 
pollination or cross pollination preparatory to fertilization, as 
described in our earlier chapters, and is followed by the fruit 
which aids in dissemination, as we shall consider in the chapter 
that follows. But with the flower, indeed, we are back to the 
fertilized egg-cell with which we began, and thus is the cycle 
completed. 

A matter of very much interest in connection with the growth 
of plants from the seedling to the adult concerns the changes in 
their tissues. The tissue of the young embryo is all capable of cell 
division (is meristematic, in anatomical language), but as the 
embryo germinates, only the tip of the root and the first bud, to- 
gether with a thin hollow cylinder of cambium connecting them, 
remain so, while all the remainder of the cells grow large, assume 
special functions, and lose their power of division. The new 
growing points as they originate, whether on stems or on roots, 
establish connections with this cambium cylinder so that to- 
gether they form one continuous system, in which all of the grow- 
ing points are connected with one another by hollow cylinders of 
cambium, and conversely, the cambium cylinder branches into 
numerous tapering tubes terminating in the growing points, as 
our diagrammatic figure illustrates (figure 139). Meantime the 
cambium grows steadily outward, as the growing points grow 
steadily onward, each forming permanent tissues behind them. 
This separation of growth and permanent tissues makes it possible 
for a plant to go on growing without limit, and were it not for the 
restrictions imposed by external physical conditions, there is no 
reason why trees should not be immortal. In this possession of 



B. Cross section through stem at j. 




C. Longitudinal section through ./). Lon|itudin*l section through 
tip of stem. tip of root 



FIG. 139. -Generalized drawings illustrating the growth system of the plant. 



363 



364 



The Living Plant 



continuously working embryonic tissues, plants are sharply dis- 
tinct from animals, all of whose tissues sooner or later become of 
the permanent kind, thus limiting their further growth. 

Although the most typical stems possess this remarkable 
cambium system, there are others which lack it. In these the 
further growth takes place by the addition of new fibrovascular 
bundles, or veins, among and outside of the old ones, so that the 




FIG. 140. Cross sections of stems of the two typical kinds, endogenous with scattered 
bundles (the Palm on the left), and exogenous with the bundles in rings (the Scotch 
Fir on the right). The matter is further explained in the text. 

fully-grown stem is composed of separated bundles scattered 
irregularly through the ground tissue, as well seen in the Corn, or 
a Palm stem (figure 140, left), or any plants of the great clas- 
sificatory division of the Monocotyledons. Observation alone, of 
these stems, conveys the impression, though a false one, that the 
new bundles originate inside of the old ones, whence they have 
been described improperly as endogenous, in contrast with the 
exogenous growth from cambium (figure 140, right). The growth 
of exogenous stems involves matters of much interest, as figure 141 



The Orderly Cycles Pursued in Growth 365 



will help to illustrate. Although 
these stems possess separate fibro- 
vascular bundles with intervening 
plates of soft tissue at the start 
(compare figure 73), the continuous 
growth of the cambium, which 
forms new duct tissue on its inner 
and new sieve tissue on its outer 
side, gradually fuses the bundles 
into one woody mass, although 
preserving, more or less perfectly, 
the intervening plates of tissue 
called medullary rays. The growth 
of the cambium, however, is peri- 
odically checked by the winter, and 
the contrast between the small 
compact autumn-formed cells and 
the large loose tissue of the spring 
(figure 139, B) gives rise to the 
familiar phenomenon of the annual 
rings, which appear also, though 
faintly and in reverse order, and 
ultimately crushed to unrecogniza- 
bility, in the bark. Most of these 
features show well in such a wood 
as that of the Oak, where the 
annual rings, and even the sepa- 
rate ducts, are easily visible to the 
eye, while the medullary rays be- 
come the broad shining plates 
which give distinction and value to 
quartered oak. Meanwhile corky 
waterproof layers are forming in 
the outer part of the bark, which 




FIG. 141. A segment, including a three 
year old fibrovascular bundle, of a 
typical stem, the Linden, showing 
the annual rings of the wood, the 
cambium cylinder, and the annual 
rings (less prominent) in the bark. 
Compare with a single bundle in fig- 
ure 139, B. (Copied from a wall- 
chart by L. Kny.) 



3 66 The Living Plant 

includes everything outside of the cambium. Though the bark, 
like the wood, thus increases in thickness as long as the tree lives, 
at least theoretically, in practice it weathers away about as fast 
on the surface as it forms inside. It will now be clear why this 
exogenous mode of growth permits the indefinite expansion of 
woody stems. 

In close relation to the age at which flower buds first appear is 
the length of life of the plant. When plants come to flower the 
season they germinate, all of the food they can make is thrown 
into their seeds, and the soft stems then die, root and branch: 
such plants are annuals. Other kinds, however, make only leaf 
buds the first season, and store up food in some underground part 
to which they die down; then this food is made use of in forming 
new stems, flowers and seeds the next season, after which the 
plants perish completely: such plants are biennials. Yet other 
kinds, in the second season, instead of throwing all of their food 
into seeds, store a part underground, die down thereto and then 
send up a new flowering stem the next season, and so on year after 
year: such plants are herbaceous perennials, which include so 
many of the favorites of our gardens. Finally, there are many 
others which do not die down to the ground at all, but harden 
their stems to wood, and thus can stand upright over winter. 
Thereafter, each season's growth, whether in length or in thick- 
ness, is built upon that of the preceding, and the structure thus 
grows both in length and in thickness as long as it lives: such 
plants are woody perennials, which are principally shrubs and 
trees. Since this method admits of indefinite increase in size, and 
since, moreover, it involves a constant rejuvenescence of the 
protoplasm (the significance of which is discussed earlier, on 
page 162), it is obvious that trees have no limit set to their growth 
by internal factors, but their maximum size is imposed by the 
action of extrinsic causes, such for example as the increasing 
difficulty of conducting sufficient water supply through the 
greatly lengthening stems. Thus, with increasing size it becomes 



The Orderly Cycles Pursued in Growth 367 

more difficult for them to transfer a sufficiency of water and min- 
erals to their more distant parts, whose vitality is thus checked. 
At the same time the increasing exposure of parts to the winds, 
and the greater leverage thus given, leads to breakage, and hence 
the admission of rot-producing fungi, which sooner or later bring 
the loftiest tree to the ground. Thus trees, though they grow very 
large, never really stop growing in size; and, moreover, they never 
grow old in the sense that animals do, but come to their end while 
their individual parts are still in full vigor. 

Such is the cycle of growth in the most highly organized plants, 
and very different it is, as the reader will have noticed, from the 
cycle displayed in the highest of animals. For animals construct 
but a single set of organs, which last without renewal through life; 
and when these have each grown to full size the growth of the 
individual is stopped, though it may live for a very long period 
thereafter. Inside of these organs the protoplasm goes on working 
without chance for rejuvenescence, and therefore gradually wears 
out and dies, thus fixing an internal limit to the length of the 
animal's life. 

Through such a complex though orderly cycle do the most 
highly organized plants all swing in the course of their develop- 
ment and growth. In tropical climates the cycle is accomplished 
without pause, except for a brief time during dissemination, but 
in temperate regions the continuity is rudely disturbed every 
year by the advent of winter, to which all vegetation must in 
some way make adjustment. One could hardly believe, a priori, 
that plants could accommodate themselves to ranges of tempera- 
ture from forty degrees below zero to a hundred and twenty 
above it; yet such is the fact. This adjustment of vegetation to 
winter introduces a secondary seasonal cycle, which has the four 
following stages: 

The Winter is the season of dormance, in which vitality is sus- 
pended. The protoplasm, giving up the most of its water, ceases 
to move, becomes hard, reduces all activities to a minimum, and 



368 The Living Plant 

goes into a resting state. Its condition in the buds, the cambium, 
and the other living parts then approximates to that of the seed. 
This is the season of gray and brown colors, which are distinctive 
of dried tissues and of the non-conspicuous protective bark and 
bud scales. 

The Spring is the season of unfolding, when the plant absorbs 
water, the sap rises, and the protoplasm awakens to a new and 
exuberant life. Then all of the structures developed in the buds 
the preceding season enlarge rapidly through their grand period, 
and unfold to soft-textured foliage and flowers, transforming the 
whole face of Nature. This too is the principal season of fertiliza- 
tion, and of germination, and of new life in various forms. It is 
the season of delicate colors, for not only is it the time of most 
flowers, and of the softest foliage green, but much of the young 
vegetation is overspread by a blush of rosy red, which perhaps is 
the protective and warming screen to the much exposed proto- 
plasm before the chlorophyll is fully made. 

The Summer is the season of accumulation. The green leaves, 
in the full vigor and strength of maturity, are engaged in the 
making of food, which passes steadily away to the places of 
storage or use, providing for current needs and preparing a new 
store for the future. It is the time of development of embryos, and 
formation of fruits. It is the season of greenness, the typical 
time of vegetation, the state that is permanent in the tropics. 

The Autumn is the season of fruition, the time of ripening 
which is always a preparation for the future. The tissues are 
strengthened and hardened; the parts to come out the next spring 
are completed in the buds and enwrapped with protective covers; 
the fruits are brought to perfection and made ready for their func- 
tion of dissemination; the leaves are emptied of their valuable 
matters and made ready for their annual fall; the protoplasm 
throughout the plant gradually yields up its water and assumes its 
resting condition. It is the season of brilliant colors, red, purple 
and yellow, a part (those of fruits) serving definite uses, but the 



The Orderly Cycles Pursued in Growth 369 

most of them (the autumn foliage), a chemical accident, though 
one with the happiest consequences to the pleasure of man. Then 
the winter comes again and the seasonal cycle is closed. 

There remains yet one other aspect of growth, and that of no 
little importance, namely, the remarkable results that ensue 
from its disturbance. All growth when left undisturbed tends of 
itself to produce symmetrical structures. In evidence thereof 
one has only to recall the superb symmetry of an Elm or a Maple 
when growing alone in a meadow, or the perfection of conical 
form in a Fir tree which springs up in some field that is abandoned, 
or the regularity in arrangement of leaves which develop within 
the protection of a bud. And the same thing can be shown very 
beautifully by experiment. Accordingly when a plant deviates 
from symmetry it is always because of disturbing influences, 
of which there are some four classes. 

Disturbance of Growth by Accidents. These are many and so 
obvious as hardly to need comment, including overcrowding by 
other plants, breaking of branches by wind or ice, destruction of 
parts by animals or Fungi, scalding of newly exposed bark by the 
sun, drying back of parts through failure in water supply, poison- 
ing by bad gases, and many others of various sorts. And it is 
important to observe that the destruction of parts of a plant by 
any of these agencies is followed as a rule by an effort at replace- 
ment and the restoration of the symmetry, as can be seen in trees 
which have lost some of their branches. 

Disturbance of Growth by Adaptive Adjustment to the Surround- 
ings. This subject received full treatment in an earlier chapter, 
where it was shown that individual plants can alter greatly the 
details of their form, size, or structure, in adjusting themselves to 
take advantage of the best conditions of their immediate en- 
vironments. It is most conspicuous in connection with adjust- 
ment to light, towards which a plant will often bend its entire 
structure; or in connection with adjustment to moisture, towards 
which an entire root system will often extend in a very unsym- 



370 The Living Plant 

metrical fashion. Such disturbances of symmetry are wholly 
natural and healthful, unlike the cases which follow. 

Disturbance of Growth by Parasitic Stimulation. The parasites 
of plants are of two general classes, Insects and other plants, 
mostly of the simple kinds called Fungi. Everybody knows 
the structures called Galls, especially common and typical upon 
Oak leaves, where they appear as rounded, almost nut-like, often 
hairy, sometimes red swellings which, when opened reveal al- 
ways the presence of a living insect larva. There are hundreds 
of kinds of these galls, very different from each other but each 
kind so distinctive that an expert can distinguish them easily, and 
even identify the insect which made them. Other galls are almost 
hair-like, others are globular swellings of very slender stems, while 
yet others include the terminal buds, and involve the leaves in 
a way to produce those compact rose-shaped structures often 
called willow-roses. They are all made in substantially the same 
manner; an insect lays its egg in the growing soft tissue, and the 
developing insect causes the plant tissue around it to form such a 
structure, and to lay up such contents, as will provide both a safe 
home and a sufficient food supply for the larva until its maturity, 
when it makes its way out and away. But just how the result is 
effected by the insect is not at all certain, whether by mechanical 
movements or chemical secretion. Nor is it yet certain just what 
the plant's attitude is towards the gall. It can hardly be true 
that the plant derives benefit from the excretions of the 'insect, 
since the original substance to make those excretions is mostly 
supplied by the plant itself. It seems much more probable that 
the plant is passively affected by the insect, which has discovered, 
so to speak, just the chemical substance or the developmental 
stimulus which happens so to fit some peculiarity of the metab- 
olism of the complicated protoplasm as to stimulate it to the 
formation of structures and substances adaptive to the uses of the 
insect. Theoretically, man ought to be able to affect plants in 
analogous ways, and it is not unlikely that the horticulture of the 



The Orderly Cycles Pursued in Growth 371 



future may embrace wonderful new vegetables or fruits produced 
by the injection of chemical substances into the young growing 
tissues of ordinary plants. 

Of somewhat analogous nature are the abnormal growths pro- 
duced by the presence of parasitic plants. A typical case is found 
in those remarkable dense growths 
of many slender upright twigs, 
found often on Spruce trees and 
commonly mistaken for nests of 
big birds. They are generally 
known as Witches Brooms, and 
are caused by the presence of a 
Fungus which sends its nutritive 
threads into the young growing 
branch, and seems to produce a 
paralysis of the delicately-bal- 
anced controlling power of 
growth. As a result all the buds 
in that region proceed to develop, 
though ordinarily they would 
mostly be suppressed, and each 
grows for itself without any ge- 
otropic correlation with its neigh- 
bors. Of precisely similar nature 
are those spiral-radiate-saucer growths on the roots of tropical 
trees, often sold to tourists as curiosities under the name of 
" wooden flowers " (figure 142). Unlike the case of the galls, 
there is no obvious advantage to the parasite in these methods 
of growth of its host, and the result seems purely incidental 
to the paralysis of the regulatory power, the direction that 
anarchy takes, as it were, when the hand of the law is re- 
moved. We see something of the very same sort in the animal 
world and especially in mankind, in tumors and other abnormal 
growths, which are likewise a continuation of growth without 




FIG. 142. A "wooden flower" from 
Guatemala, one-third the natural size. 
It is explained in the text. , 



372 The Living Plant 

control. From such elaborate structures as the Witches Brooms 
there are all grades downward to those so inconspicuous as hardly 
to attract notice, including some very simple kinds of excrescences. 

Results very similar to these may be caused by external me- 
chanical injuries. Thus, when tree trunks are injured in the cam- 
bium, this also loses its regularity of growth, and becomes thrown 
into various contortions, producing gnarly fibrous growths which 
often appear as large burls on the trunks. Moreover, the injured 
cambium at times attempts to put out adventitious buds, which, 
forming in large numbers but without proper control, just about 
keep pace with the expansion of the trunk. The resultant masses 
of wood show the characteristic rings of buried branches, often in 
patterns displaying great beauty, of which the Birdseye Maple 
affords a conspicuous example. 

Disturbance of growth by Internal Causes. In addition to the 
external and visible causes which throw growth into confusion, 
there are others which appear to be internal. One of the simplest 
cases is that in which the correlation between the different parts 
of a plant becomes weakened or lost. This correlation is beauti- 
fully shown in the familiar fact that if the young tip of the main 
stem of a Spruce or a Fir tree be cut away, one of the nearest 
branches will grow up and take its place, although, had the tip 
remained, that branch would have grown like its neighbors 
horizontally outwards. It is indeed this correlation of the geot- 
ropism of the branches which makes possible the symmetrically 
radiate shape of a tree, each branch as it grows assuming the 
correct geotropic angle to form either the cone or the ball of 
foliage as the case may be. Now in old trees this correlation is 
sometimes lost, perhaps through the interruption of the pro- 
toplasmic connections along the stem, and then each new branch 
grows upward precisely as if it were the only main stem, as our 
accompanying figure illustrates (figure 143). Obviously such 
cases are related in method with the Witches Brooms and the 
like, earlier mentioned. Somewhat the same thing occurs in 



The Orderly Cycles Pursued in Growth 373 




the case of branches, or sprouts, that spring anew from old 
trunks. 

More profound disturbances, also of internal origin, result in 
the formation of monstrosities, or in common language, "freaks." 
They are really rather common, and at- 
tract much notice because of their oddity. 
In general they may be distinguished 
from the effects of fungus or insect action 
by the fact that although they look odd 
they also look healthy. Very typical are 
those called fasciations, which arise in 
this wise, that, from causes unknown, 
some terminal bud, instead of develop- 
ing as a single cylindrical structure, be- 
comes partially split into a number of 
points, which usually spread out like a 
fan, and produce a flattened or corru- FI O . 143. AU old Apple tree, 
gated stem with many little terminal %$ k jtf&.< 
points. A remarkably fine example of a J )een los , t paving each bud 

* t t free to develop as if it were 

f aSCiated Pineapple is Shown by the aC- the only one. (Traced from 
. , ,(* i A A\ i a photograph.) 

companying picture (figure 144), and 

most people have seen fasciations in Asparagus, Hyacinths, or 
other strong-growing plants. Fasciations are also the basis of 
the crested forms of Cactus and other plants, and give the 
' ' cockscomb " to the plant of that name, in which, as in some other 
cases, the condition is hereditary. Fasciations can also be pro- 
duced, by the way, by external injury, such as the bites of some 
insects, though when produced in such manner they are not hered- 
itary. They are of all degrees of complexity, down to a simple 
forking of the growing point, which may sometimes result in the 
formation of double fruits, though these are more often the result 
of the fusion of two buds in a sort of natural grafting. It is 
obvious that such fasciations come very close to the condition 
which originates the Birdseye Maple, or rather that the latter 



374 



The Living Plant 



in reality is a kind of fasciation. It is perfectly impossible to 
draw any sharp line between these different forms of clustered 
abnormal growths, or between external and internal causes of 
their formation. 

Somewhat similar is the origin of twisted stems or torsions. 
These occur in small herbs, but are often seen to perfection in 
dead standing trees, or even in the logs of fence rails; but here the 




FIG. 144. A fasciated Pineapple, resulting from causes explained in the text. 

process is hardly abnormal, since, as seems likely, the twisting of 
the cambium cylinder, to which it is due, is a result of normal 
growth processes in the plant. 

A second common form of monstrosity is that known as pro- 
liferation. Sometimes the stem of a pear, or a strawberry (fig- 
ure 145) grows on beyond the fruit, producing there a cluster of 
leaves. In one way these cases are easy to understand, for they 



The Orderly Cycles Pursued in Growth 375 




are simply instances wherein the stem, which ordinarily ceases to 
grow in a flower bud, keeps on growing just as it does in the leaf 
buds, though why it should do so is not as yet known. We have 
a partial case of the same thing in the navel orange, in which the 
receptacle or stem grows part way up through 
the fruit and makes there a second series of 
carpels, which constitute the secondary orange 
within the navel; and the same thing carried 
farther in apples sometimes gives us a two- 
storied fruit. 

As to other monstrosities they are legion, 
enough, indeed, so that their mere synoptical 
description suffices to fill large volumes devoted 
to the subject. Flowers double profusely; 
leaves instead of their characteristic flatness 
exhibit often the form of a pitcher or cup; 
pistils become open leaves, exposing the ovules, 
which themselves become altered at times to 
tiny leaflets; apples and cucumbers produce 
leaves on the sides of their fruits; flowers become green, and 
bracts of the stem assume the colors of flowers; and so many 
other alterations of form, color, size and regularity occur that 
it sometimes seems as if every deviation from normality struc- 
turally possible in any and every part of the plant became some- 
time or other actually realized in fact. Some of these mon- 
strosities are hereditary, though mostly they are not, and many 
of them could be propagated by grafting if it were thought 
worth while. It is evident that they merge without break over 
into those extreme variations which are called, horticulturally, 
sports. 

Monstrosities are sometimes reversions to an ancestral condi- 
tion, and formerly they were thought always to be so. Hence 
they were supposed to throw light upon evolution and classifica- 
tion, an idea embodied in Goethe's saying that "by her mistakes 



Fio. 145. A Straw- 
berry, in which the 
stem that forms the 
fruit has grown out 
beyond it into a 
leafy branch. (Copied 
from Masters' Vege- 
table Teratology.) 



376 



The Living Plant 



Nature betrays her secrets/ 7 But so many monstrosities are 
known which cannot be interpreted as reversions that we must 
consider them rather as results of disturbance of the growth 
processes, though we have no idea as to the ultimate causes. 
They can mostly be interpreted in terms of failure of action on 
the part of the suitable stimuli. Thus, in green roses, the stimuli 




FIG. 146. Typical examples of water-rolled weed balls, photographed about two-fifths 
the natural size (the squares of the screen are each one centimeter). The largest is 
composed of various Algar, the next in size, of Spruce needles; the next, o! Pipe-wort; 
the oval one, of hair; while the composition of the fifth is uncertain. 

which started the formation of a flower bud instead of a leaf bud 
w r orked properly, but those which controlled its farther develop- 
ment did not. But as to the causes of such failure of stimuli we 
have no information. 

In connection with plant-structures of odd mode of growth, we 
must take note of one having a very different character. On 



The Orderly Cycles Pursued in Growth 377 

some sandy shores of the sea, or of freshwater lakes, the visitor 
sometimes finds balls of vegetable matter a few inches through, of 
a roundness and symmetry wholly surprising. Naturalists once 
thought them a species of seaweed, while peasants and doctors 
ascribed to them medicinal virtues. In reality they are nothing 
other than masses of the fibrous parts of half-decayed plants, 
matted, compacted and rounded by the gently-rolling action of 
the underwater parts of waves acting over smooth sandy bottoms. 
They are made up of the most diverse materials, plant-fibers, 
fine seaweeds, needles of pines, spruces and hemlocks, and even 
such adventitious materials as shavings and hair. A number 
of different kinds are well shown on the accompanying plate 
(figure 146). These are not the only kinds of vegetable balls 
that are known; others, sometimes called bezoars, are formed by 
the rolling and matting of indigestible fibers in the stomachs of 
cattle. Somewhat analogous is the curious algal paper, sometimes 
formed by the drying of continuous masses or sheets of matted 
Algae left by the falling water of lakes. And doubtless there are 
other structures also, which appear to be products of some special 
mode of growth while in reality they are merely a result of the 
play of natural forces. 




CHAPTER XV 

THE MANY REMARKABLE ARRANGEMENTS BY 
WHICH PLANTS SECURE CHANGE OF LOCATION 

Dissemination; Fruits 

]NE of the most obvious and consequential of the facts 
about the typical plants is their sedentary habit; 
they are rooted immovably in one spot. Yet all of the 
kinds are able at some stage in their lives to change their 
locations, though the methods whereby this is done are most di- 
verse, as the following pages will abundantly demonstrate. 

We make sure, first of all, why a change of location is needed. 
To take the most obvious reason, it is evident that if all of the 
seeds that any plant ripens were to fall direct to the ground and 
germinate there, a jungle would result so dense that few, or per- 
haps none, of the plants could survive. A power to spread from 
their parents is therefore essential in order that individuals may 
find space in which to develop. But there are reasons, as well, of 
a secondary sort. Thus, any kind of plant, whether because it 
exhausts from the soil some material it needs in its growth, or 
because it imparts to the earth some excretion injurious to itself, 
cannot grow a very long time in a single location without deterio- 
ration of vigor. Again, in an ever-changing world, it is an ad- 
vantage to any species if it can leave a situation becoming less 
favorable for its life and migrate to some other that is becoming 
more favorable. Furthermore, it seems true of plants as of men 
that an occasional change to different surroundings acts stimu- 
latingly upon health and adaptability, and therefore is distinctly 

378 



How Plants Secure Change of Location 379 

advantageous in the struggle for existence. And other reasons 
exist, of lesser weight, which combine with those given to explain 
both the need and the value of a change of location. 

The methods whereby plants secure this change of location are 
many and various, but fall somewhat naturally under these 
divisions : 

1. Independent Locomotion 

2. Extension through Growth 

3. Projection by Elastic Machinery 

4. Waftage by Winds 

5. Flotage upon Water 

6. Carriage by Animals 

The roll of these methods will recall to the reader our dis- 
cussion of the use of the very same ones in connection with cross 
pollination. They are, in fact, substantially the same, as would be 
inferred from the similarity of the problems presented to the 
plant in the two cases. The chief differences are connected with 
the greater difficulties of cross pollination (for here a definite 
goal as well as a definite starting place is imposed), and with the 
extreme fineness and lightness of pollen, which makes its pro- 
pulsion from plant to plant impracticable. But the identity of 
method in the two processes should not be permitted to create 
any confusion between them in the mind of the reader, who 
should keep very clearly in mind the totally different -object in 
the two cases. 

1. Independent Locomotion. Although none of the higher, or 
familiar, plants possess this power, it is well developed in the sim- 
pler kinds which lack the firm cellulose skeleton; and the method 
thereof is precisely the same as is used by the simpler animals. 
Thus, some kinds creep, as in the case of the Slime-molds (or 
Myxomycetes), with which the reader has already made acquaint- 
ance in the chapter on Protoplasm; these opaque-white gelatinous 
masses are sometimes seen in damp places, on decaying wood, 
wet earth, or neglected flower pots, where they creep about, 




380 The Living Plant 

though slowly, by the simple device of directing their protoplasmic 
streaming in one constant direction, precisely as is the habit of the 
much smaller Amoeba among animals (figure 147). Other plants, 
again, or their reproductive spores, can swim freely in water, in a 

manner so like that of animals that 
they are known as "animal spores/' or 
zoospores; and these are very abundant 
and characteristic in the Algae or Sea- 
weeds (figure 94). The motion is ef- 
fected either by the action of innumer- 
able cilia, tiny hairs which all in unison 

FIG. 117. An Amoeba, greatly , , . 

magnified; a creeping organism beat the water more strongly in one 
mentioned in the text. direction than the other, or else by 

flagellae, which are structures suggestive of tails, except that 
instead of pushing the spore, they pull it behind them by an 
action the reverse of the one used in the tail of a fish. These 
movements of flagellae and cilia, by the way, depend upon the 
power of contractility in protoplasm, a form of the motility 
which has already been described as one of the physiological 
properties of that substance; and this same contractility, also, 
is the basis of the muscular mechanism of the higher animals. 
Other kinds of water plants, notably the Blue-green Algae, make 
use of vibration of their rod-like bodies, securing their movement, 
I suppose, in essentially the same way that a piece of flexible steel 
is shot through the air after having been bent between thumb 
and forefinger and then quickly released. Other kinds push out 
protoplasmic threads, which work against the bottom, as is the 
way with the Diatoms, those tiny plants whose wonderfully- 
sculptured shells are the favorites of every happy possessor of a 
first microscope. In all of these modes of locomotion, the re- 
semblance to animals is not accidental, but a persistence from an 
ancient condition in which the two kingdoms were one. 

2. Extension through Growth. The stems of the higher plants, 
as the reader will recall, are usually so made that elongated inter- 



How Plants Secure Change of Location 381 

nodes separate the bud-bearing nodes, which, moreover, in most 
soft-textured plants, can readily strike root. It is plain that if 
stems are sent out horizontally in such manner as to touch the 
ground, the nodes at their tips may strike root and send up new 
shoots, thus originating new plants at some distance from the 
parent, from which they will later be cut loose by the death of the 
intermediate stem. Plants have not been slow to take advantage 
of the possibilities of this method. Everybody knows the typical 
case of the Strawberry, with its long slender runners which bear 
tiny plants at their tips; and the same thing is found in the House- 
leeks, and others too many to mention. Some plants send the 
steins underground, after the mariner of roots, and form new 
plants, called suckers, at places not possible to predict; and this 
makes them hard to exterminate, as in the case of the Yarrow, and 
some other weeds of pertinacious character. Suckers, by the way, 
spring also from roots, some kinds of which can make buds, es- 
pecially when injured; and this is the way with some fruit trees, 
like Apples. In a similar manner, the horizontally-radiating 
underground equivalents of roots, the mycelial threads, of some 
Mushrooms, send up the new Mushrooms at so regular a distance 
from the parent as to form a conspicuous ring, whose name " Fairy 
Ring," implies an ancient belief as to its origin, (figure 148). 
Again, among the more familiar plants, there are shrubs, of which 
our Briars and Blackberries are examples, with stems so slender as 
to curve over and bring their tips to the ground, where they take 
root and produce new plants, known as stolons; and these con- 
necting stems for a time form a trap for the feet of the unwary, 
giving name to the various shrubs called Hobble-bush. The Walk- 
ing Fern gives another example of this method. 

There are plants, however, in which the main stem itself creeps 
on or just under the ground, striking root and sending up shoots as 
it goes, thus spreading its own growths to new ground. This is 
the way in the Grasses, whose creeping stems run and branch so 
freely, and interlock so closely, that they form the dense mats we 



382 



The Living Plant 



call turf. Our native ferns, as well, have stems that creep, and 
send up the beautiful fronds from new soil. There are other 
plants, like Solomon's Seal, which grow onward under ground 
year after year, the old parts dying behind as the new are devel- 
oped in front; and such plants may wander a considerable dis- 
tance through the woods, carrying their new branches ever into 
new ground. In the tropical forests there are epiphytes which 
wander in this manner over tree trunks, and certain undergrowth 
kinds which grow forward a little on stilt-like aerial roots. 

&1 ,!,,**!%_* , f ' ft 




FIG. 148. Fairy Rings, of Mushrooms, originating as explained in the text. Three com- 
plete rings and a partial one appear in the picture. (Reduced from Kerncr's Pflan- 
zenlcben.) 

Roots possess a certain power of shortening their length during 
later growth, and advantage thereof is taken by some bulbous 
plants, like the Tulips. New bulblets are formed in the axils of 
the scales of the old ones, and then are pulled an appreciable dis- 
tance from the parent by the shortening of their own radiating 
roots after these have become fixed in the soil. And there are 
other minor ways in which growth helps to spread plants, though 
I think the aforementioned include all of real consequence. 



How Plants Secure Change of Location 383 

It is obvious that the ways described in this section, while 
efficient so far as they go, secure no wide spread for plants; they 
are, indeed, supplementary, or extra, methods which happen to 
be rendered available by some peculiarity of growth or habit in 
the plants concerned, all of which 
possess also the far more efficient 
methods we are now to consider. 
As to these latter, they depend 
in reality upon a single principle. 
Forbidden by their mode of life 
to move when adult, plants have 
taken advantage of that stage in 
their lives when they are small 
and therefore easily transportable, 
the stage of the embryo. This 
embryo, with its vitality sus- 
pended for a time, together with 
its store of food substance and 

protective COats, Constitutes the FIG. 140. Pod of a Vetch, explosively 
o, ! i 1 j f ji propelling its seeds. 

Seed, which is severed from the 

plant and can then be transported in various ways as we shall 

now proceed to consider. 

3. Projection by Elastic Machinery. The appreciable size and 
weight of seeds makes possible their projection to some dis- 
tance by a sudden application of sufficient power; and this fact 
has been readily turned to use by plants for their dissemination. 
The propelling machinery is variously made. In some kinds of 
seed-pods, definite bands of cells ripen in a state of stretched 
tension, which presently becomes so great that the pod bursts 
suddenly, hurling out the loose-lying seeds to a distance of several 
feet. In the Vetches, the two halves of the pea-like pods twist 
suddenly apart in opposite directions (figure 149); in the Wild 
Geranium, the ripening styles suddenly curl up from the length- 
ened receptacle (figure 150); the valves of the capsules burst 





384 The Living Plant 

apart explosively, shooting out the seeds, in the Castor Bean, 
the Witch Hazel, the Acanthus (figure 151), or the West In- 
dian "Sand Box," whose report is said to rival that of a pistol. 
In all of these cases, the propulsion of the seeds may be seen, 

heard, and even smartly felt by the 
reader, if, when the pods are near 
ripeness, he will bring them to the 
room where he spends most of his 
time. In the Violets the sides of 
the ripening pods come to press 
harder and harder upon the smooth 
seeds which are held in the angle 
between them, until finally the seeds 
are shot out of the pods in precisely 
the same way that a smooth bean 

Fio. 150. The seed-propelling fruit of . , , . - , . 

wild Geranium, explained in the or a nut can be shot from between 

toxt ' the tightly-pressed fingers (figure 

152). In all of these cases, the seeds show approach to the 

qualities best in all shot, they are round, smooth and relatively 

heavy. 

Instead of the springing force of elastic dry tissues, some plants 
make use of turgescence, that is, of the pressure developed by 
tensely-gorged cells against lines of a weaker sort, ending in ex- 
plosive rupture and flight of the seeds. This is very well known 
in the fruits of the Jewel-weed, called also Touch-me-not, a 
common wild plant which takes its name from the habit. In the 
descriptively-named "Squirting Cucumber" of the East, the 
entire pulpy contents inside of the firm-skinned fruit ripen so 
turgidly that pulp and seeds together squirt out to a distance 
when an outlet is made by the breaking of the fruit away from its 
stem (figure 153). e 

Related to these ways in its principle, though differing much 
in detail, is the method used in those cases where small round 
and relatively heavy seeds come to lie loosely in open-topped 



How Plants Secure Change of Location 385 



capsules on long stiff-elastic stalks. When the stalks are shaken 
by gusts of wind, or the impact of animals passing, the seeds are 
thrown out by the movement, especially the jerky recoil. The 
exit of the seeds from some of these pods lies along smooth 
grooves so placed as to 
guide the seeds at an 
angle best for their flight 
to a distance. These fea- 
tures appear well in the 
Poppies (figure 154), 
upon which observation 
and experiment are easy ; 
but it really is one of the 
commonest of the modes 
of dissemination, prevail- 
ing through several large 
families of plants, nota- 
bly the Figworts, Bell- 
worts, Primroses and 

Pinks. And Other metll- FIG. 151. The pods of Acanthus explosively pro- 

ods of projection occur, jccting its sccds> 

as the reader may see for himself in the field, or find described in 

the works on the subject. 

A special form of projection through movements of ripening 
tissues is shown by those seeds which are pushed along the ground 
by movements of hygroscopic hairs. The causes of hygroscopic 
movements were considered in the chapter on Absorption; and 
it will here suffice to say that some tissues, by absorbing moisture 
from the air, or giving it up thereto, can swell and twist very 
forcibly, though not suddenly. In some Clovers hygroscopic 
hairs are so placed in conjunction with backwardly-directed 
parts, which act as " chocks," that every movement of the hairs 
pushes the seed along the ground (figure 155). The arrangement 
is yet better in the curious "living Oat" (Avena sterilis), which can 




The Living Plant 



creep appreciable distances within a few hours, and which, when 
placed with clothes in a drawer, burrows among these in a fashion 
quite uncanny. Hygroscopic movements also aid dissemination 
indirectly; for hygroscopic hairs, or equivalent structures, in some 





FIG. 1.52. The pod of a Violet projecting 
its seeds by a method explained in the 
text. 



FIG. 153. The Squirting Cucumber, pro- 
jecting its seeds as explained in the 
text. 



Orchids, Mosses and other plants, push seeds or spores from the 
interior of the capsules to the surface, where they can be reached 
and transported by action of the wind. Hygroscopic movements 
have also a part in the final bursting of seed pods in some of the 
cases described a page or two earlier. 

But the method of projection, though effective in principle, 
has marked limitations, since the maximum distance to which 
seeds can be thrown, no matter how great the power may be, does 
not exceed a dozen or two feet. This is enough for very small 
plants and limited spread, but does not suffice for much larger 
kinds, or a wider dispersal. It is fortunate, therefore, that a 



How Plants Secure Change of Location 387 

method more effective is available. This method, in brief, is 
this, the provision of appliances which ensure that the seeds 
shall be carried away by moving agencies that exist in the world 
around. These agencies are principally three, winds, water- 
currents and animals. There is also a fourth, gravitation, 
but it acts in a profitless direction, although indirectly it con- 
tributes some aid to dissemination by making round seeds roll 
down slopes, and causing elastically-walled heavy seeds, such as 





FIG. 154. A pod of a poppy, showing 
openings for exit of the seeds. 



FIG. 155. A fruit of Clover, showing the 
hygroscopic hairs. 



many trees have, to rebound from hard surfaces with a force 
which must oftentimes remove them considerably away from the 
plant that produced them. The other three agencies, however, 
are vastly important in dissemination, as the following descrip- 
tions will amply attest. 

4. Waftage by Winds. Of the motive forces of nature, winds 
are one of the most ubiquitous, and the easiest of all for plants 
to make use of. They occur in all grades from the wildest of gales, 
creating disturbance through hundreds of miles, down to the 
faintest of zephyrs confined to a limited region; and they include 
as well those upward currents of air which rise over heated places 
in summer to a height where wide-ranging breezes prevail. To 
utilize these winds for their spread, plants have only to attach 



388 The Living Plant 

to their seeds such devices as shall make them expose a great 
surface in proportion to weight, and this they have done in mani- 
fold ways. 

The simplest way of causing a seed to expose much surface to 
wind lies in the addition of a broad flat sail, or a wing. Everybody 
knows the seed of the Maple, with the lengthened wing growing 
out from the wall of the fruit (figure 156), and the Elm, with a 
similar wing except that it encircles the fruit. The conspicuous 





Fio. 156. Winged fruit 

of a Maple. FIG. 157. Winged fruits of the Linden. 

way in which these seeds in their season are blown about our 
streets proves the efficiency of the arrangement. The seeds of 
the Linden, or Basswood, are likewise transported by a very fine 
wing, made from a bract grown fast to the stalk (figure 157), 
while in Pine and Catalpa the wings grow out from the coats of 
the seed. These are representative examples; and there are 
others as well, but less common, in which the wing is supplied 
by calyx or corolla. 

Acting like the wings, and in some ways still more simple and 
effective, are large bladders, in which the seeds lie. Some ap- 
proach thereto is made by those kinds of the Pea Family which 
have pods greatly swollen but very small seeds; but it reaches 
more typical development in cases like the Bladder Nut, where 



How Plants Secure Change of Location 389 

the ovary forms a very loose envelope, or in some Orchids, where 
it is made from a greatly inflated seed coat (figure 158). In 
all of these cases the principle is the same, that of a great spread 
of surface accompanied by very light weight. 

Another fine method for giving much surface, consists in the 
provision of long soft hairs, or plumes; and seeds displaying this 
arrangement are plenty. The Cotton seed, for example (fig- 
ure 159), develops hairs of such number and length that they 
serve not only to spread it afar under action of wind, but prove 




FIG. 158. The 
seed of an Or- 
chid, showing 
through i t s 
loose h 1 a ri- 
der-like coat. 




FIG. 159. A cotton seed, with 
its long soft hairs. 



FIG. 160. A 
plumose fruit 
of Clematis. 




FIG. 161. The 
parachute 
fruit of the 
Dandelion 



incidentally a great utility to man, since they yield him the fiber 
for the commonest of all of his fabrics. The familiar silky plume 
of the Clematis (or Virgin's Bower) is made by the outgrowth of 
hairs from the style (figure 160); the parachute plume of the 
Dandelion from the calyx; the soft tuft of the Milkweed from the 
seed coat; the nebulous mass of the Smoke bush from stalks of 
unfruitful flowers. The phrase "parachute plume" used above 
was carefully chosen because of its suitability. For in the Dan- 
delion (figure 161), and some other plants, the plumes are spread 
out horizontally, and keep that position in flight because of the 



390 The Living Plant 

weight of the seed which hangs some distance below. Thus the 
seeds can be lifted over the tops of the trees by those currents of 
air which rise upward from places that are heated in summer, 
whence the wind may transport them to far distant parts. It is 
largely, no doubt, because the Dandelion, and its relatives in the 
family Compositae, have developed so efficient a mode of wind 
transport, that they constitute the largest and most widely 
diffused of all the existent plant families. And it is also of interest 
to note that these wind-carried seeds exhibit a very effective 
secondary adaptation to wind-transport, namely, great lightness 
of build, which even extends to the employment of oil, a lighter 
material than starch, as food in reserve for the embryo. And 
other modifications of this principle of plume-transport exist, 
as the reader may learn from his own observation, or the several 
good books on the subject. 

If one compares the habits of plants whose seeds are winged or 
are plumed, respectively, he will find that in general the winged 
seeds belong to trees and the plumed seeds to herbs. It is easy to 
imagine a reason for this. Plumes are a better lifting device than 
are wings. The extra height and better exposure to wind of the 
tree gives its winged seeds a start which is ample for transport to 
sufficient, if not to the greatest distances. Moreover, some tree 
seeds possess such a relation of weight to wing form that they do 
not fall directly to earth, but only after whirling through long 
spirals, thus giving the wind a longer action upon them. But 
with herbs in their low sheltered positions, the wing would be far 
less efficient; and the lifting action of plumes is a notable ad- 
vantage. The plume is actually a better device than the wing, 
and there is reason to think it a later evolutionary development; 
for our modern herbs, I believe, appeared later in time than our 
trees. 

There is still another method, quite different in principle, of 
increasing the surface in proportion to weight. It consists in 
excessive reduction in size. It is a mathematical fact that as a 



How Plants Secure Change of Location 391 

sphere or other rounded body diminishes in size, its bulk, and 
therefore its weight, diminishes far faster than its surface; or, in 
other words, the smaller such a body becomes the more surface 
does it spread in relation to weight. A body has only to reach a 
certain point of smallness, therefore, when the very slightest air 
movements are enough to blow it away, and to keep it suspended 
indefinitely in the air. This is the reason that dust floats as it 
does; and amongst this dust, and a part of it, float the spores of 
Bacteria, Molds, Yeasts, Ferns and other spore-bearing plants, 
which depend on this method for their dissemination. There can 
be little wonder that such plants are found so widely distributed 
when we remember how far this dust can be carried by any sum- 
mer breeze. Among seed-bearing plants, however, the habit of 
forming a many-celled embryo before separation of the seed 
from the parent plant, makes the seed too large for this method 
to be used, though in some Orchids the embryo formation is post- 
poned, leaving the seed small enough, especially when a loose 
open sac is added, to be transported in this manner. 

The reader who is versed in morphology will observe that I 
often ignore the distinction between the seed and its accom- 
panying fruit. From the point of view of the principle and effi- 
ciency of dissemination, it makes no particular difference whether 
the disseminating mechanism is formed from a part of the seed 
itself, or from the associated receptacle, ovary, style, calyx, or 
corolla. And if one asks why a particular plant forms its wing or 
its plume in this way, and another in that, we can only reply that 
herein lies another illustration of the first law of adaptation, 
that a new structure when needed is formed from the part which 
happens to be most available for the purpose, and sometimes that 
part is one thing and sometimes it is another. Next after the 
seed itself, however, the disseminating mechanism is most often 
constructed from the part next contiguous, the ovary; and the 
frequency of its use for this purpose has caused its retention with 
the seeds long after the other parts have fallen. It is this per- 




392 The Living Plant 

sistent ovary, modified to aid dissemination, and often accom- 
panied by contiguous parts of the flower, which constitutes the 
fruit of the plant. 

For the sake of completeness I should add yet another to the 
methods by which the wind aids dissemination, although it is 

only of minor importance. Not 
only seeds, but some other parts 
of plants which are capable of 
growth, are also transported by 
winds, especially when these rise 
into gales. It is thus with some 
leaves, in Begonias and Life-plants 
(Bryophyllum) ; joints of stem in 
some Cactuses: buds or bulblcts in 

FIG. 162. The Rose of Jericho, ex- ' 

plained in the text. (Reduced from SOme SeduHLS and LillCSJ the brittle 
Kernel's Pflanzenlebcti) . . . f -TY^H rr,i ,. . .-, , 

twigs of Willows. The fact that in 

such cases the transport is incidental rather than adaptively de- 
veloped makes it none the less real; while moreover, the very acci- 
dentality of the method illustrates to perfection the way in which 
many, and perhaps most, adaptations begin. Again, some kinds 
of plants that live in open dry places roll their branches inwards at 
times to form a kind of ball, and in this state may be blown from 
their anchorage and sent rolling across plains or the frozen snow, 
to take root again in new places, often scattering their seeds or 
spores as they go. Such plants, called " tumble-weeds" and es- 
pecially characteristic of prairies or plains, are well exemplified in 
the Russian Thistle, a troublesome new weed of the west, the Rose 
of Jericho, mentioned at times in the Scriptures (figure 162), and 
the Resurrection Plant of the southwest. Sometimes it is not the 
whole plant but only its fruit-cluster, as in members of the Parsley 
Family, which is thus broken loose and sent rolling away. A 
simpler method is displayed in those flat pods, such as some 
Locusts possess, which, curling into loose spirals that catch every 
wind, are rolled over smooth ground or the snow. 



How Plants Secure Change of Location 393 

5. Flotage upon Water. When water moves onward in currents, 
whether merely those minor and temporary kinds made by winds 
in their sweep over lakes, and by rains in the rivulets they cause, 
or in the mighty and permanent streams of river and ocean, it 
forms a good agency of transport even though less widely useful 
than winds. 

With water-currents, as with others of the methods, some 
transport is incidental, as when floods tear out whole plants from 
the banks and leave them to grow anew in another location when 
the waters subside, or as happens when rivers sweep broken 
twigs of Willow to new places where they readily strike root. 
Again, rain drops splash out from open capsules, spores or small 
seeds which are carried away in rivulets to places where they are 
left in dampness good for their germination. Thus also are 
carried little buds (gemmae) of the Liverworts, and probably the 
axillary bulblets produced by several kinds of plants. Besides, 
most wind-scattered seeds are so light that they float well upon 
water, and thus effect a still wider transportation. But these in- 
cidental methods are insignificant in comparison with those which 
are secured by adaptation in the plants. 

In the first place, some kinds secure transport by water-currents 
through their very habit of life, which indeed may be partly de- 
termined to this end. Such are the 
free-living submerged Algae, which in- 
clude vast numbers of the simpler 
kinds of Seaweeds, and the free-float- 
ing plants like our Duckweeds (figure 
163) and the Water Hyacinth, with Fio 163 _ The Duekweodt a float . 
similar kinds of the tropics. And a in plant. (Copied from the 

. - Chicago Textbook.) 

modification of this habit is found 

in some sorts of our own, like some Watercresses, which are only 
lightly attached and are readily moved to new places; while many 
kinds of our water-weeds form naturally-detachable buds which 
are easily floated afar. 




394 The Living Plant 

But the most perfect transportation by water is found in those 
cases where seeds are adapted expressly to this method, which 
requires some kind of a float, and a power to resist decay for a 
considerable time. Thus in the African Lotus, or Nelumbium 
(figure 164), the great top-shaped and air-filled receptacle, well 
known from its conventionalized use in the art of the East, forms 
a very effective float for the seeds which are dropped here and 





Fia. 165. The seed of 
a water-lily, with its 
Fio. 164. The floating receptacle of Nelumbium, flotation bladder, 

showing a part of the seeds. (Copied from Gray) . 

there as it goes. In some Water-lilies, the float is an air-filled 
bladder formed by a loose seed coat around each single seed 
(figure 165) ; and in other commoner kinds the float is a swollen 
wall of the ovary. In the Cocoanut the great air-filled husk is a 
development of the ovary, and so perfect a flotation device that 
this plant forms the chief one of the palms that rise in the tropic 
isles throughout the seven seas. So perfect, by the way, is the 
power of this husk to resist decay in the water, that a cordage, 
called coir, is made therefrom for special use where resistance to 
decay in salt water is particularly needed. And by analogous 
methods other seeds as well have been carried over vast reaches of 
ocean. 
6. Carriage by Animals. Most ubiquitous of all of the moving 




How Plants Secure Change of Location 395 

agencies of nature, so far as utilization by plants is concerned, 
are animals, which forever are roaming among plants in their 
search for food or for shelter. And in general where plants are 
most plenty, there animals too most abound. There is, by the 
way, a kind of poetical justice in plants 
making animals do service for them as 
some return for the priceless benefits they 
confer upon animals. 

The ways in which animals are made 
to cooperate in the dissemination of " ''" ' ''' 

plants are various. In the first place, as 
in case of other methods, some transport 
is incidental, that is, it occurs without Fic . 100. The hooked fruits 
the existence of any particular adaptations of Burdock - 

thereto. Thus the seeds of some water plants are carried vast dis- 
tances embedded in the mud which adheres to the feet of the larger 
and wide-ranging water birds, some of which have been shot with 
such seeds attached to their feet or their feathers. Again, some 
heavier seeds, such as nuts, are carried away by squirrels or birds 
to be eaten elsewhere or stored up for winter; but some are dropped 
on the way and others never are used, so that they come to grow 
in new places. Probably also the scattering of spores of Mildew, 
or other leaf Fungi, by temporary adhesion to the slimy bodies of 
snails is of similar nature. 

Turning now to the definite adaptations which fit seeds for 
transport by animals, we find first of all a simple and obvious 
method in the provision of hooks or other arrangements suitable 
for attachment of seeds to fur or to feathers. Everybody will 
recall the case of the close-clinging Burdock (figure 166), while the 
Cocklebur and the Agrimony are equally efficient. Some striking 
examples occur in the plants of great plains, where large animals 
are especially abundant, as for instance the Unicorn Plant of the 
west (figure 167) which catches in the tails of horses, and the 
Grappling plant of South Africa (figure 168), which entangles 



396 The Living Plant 

itself in the fur of lions. But innumerable small plants use this 
method, as one's clothing bears visible testimony after rambles 
through fields in the autumn. The hooks are most diverse in 
form as well as morphological origin, some coming from seed 
coat, some from ovary, some from calyx, some from bracts, no 
doubt in each case along the lines of development that were 
easiest at the moment. However tightly these hooks may cling, 





FIG. 108. The Grappling plant, ox- 
plained in the text. (Copied from 
FIG. 167. The Unicorn plant, ex- Miss Stoneman's Plants of South 

plained in the text. Africa.) 



the seeds sooner or later fall to the ground, either brushed away 
by contact with some hard object, or else dropped when the hair 
of the animal is shed. Nor is the employment of hooks confined 
only to seeds, for they exist also on some separable joints of small 
Cactus, or the slender stems of the Bedstraw or "Tear-thumb," 
both of which secure some transport through the contact of 
wandering animals. 

Hooks are efficient with fur but less so with feathers, to which 
some adhesive material is better adapted. Thus, seeds which are 
carried by birds commonly possess a covering of mucilage, as in 
very many water plants; and so effective is this method, in con- 
junction with that where the adhesive is simply the mud of a 
pond, that water plants are among the most widely-distributed 



How Plants Secure Change of Location 397 



of all living things, some kinds actually occurring in all of the 
continents. Especially effective is the very sticky "bird-lime," 
formed by Mistletoe berries and many other parasites; such seeds 
adhere to the feet or feathers of birds and thus obtain attachment 
upon trees, the only positions in which 
they can grow. Some low-growing 
herbs like the Twin-flower (figure 169), 
attain the same end by the possession of 
adhesive glands on the fruit. 

There remains but one other method of 
utilizing animals in the transport of seeds, 
and that is the most striking and im- 
portant of all. It consists in providing 
the seed with some form of indigestible 
covering, surrounding the same with a 
nourishing and appetizing pulp, and giv- 
ing the whole a bright color which con- 
spicuously displays its position. Such 
fruits are then eaten by animals, and the 

seeds pass through their bodies unin- Flt; 100 ._ Tho g i andu i ar . ad . 
jured, after an interval that usually en- 
sures their discharge at a place con- 
siderably distant from where they were eaten. This without 
doubt is the explanation of the existence and characteristics of 
colored and edible fruits in nature; and so abundant and familiar 
are they that we need hardly cite any examples. So common, 
indeed, are edible fruits, and so effective their use, that this 
method of dissemination must rank very high among the modes 
of plant transport, and is second, if to any, only to wind waftage. 
To this method of seed transport, birds are better adapted than 
other animals, since their smaller size makes it possible to attract 
them with not too lavish provision of pulp, and their very active 
habits ensures their movement over considerable spaces. Ac- 
cordingly, colored fruits are especially abundant on trees, shrubs, 




hosive fruits of the Twin- 
flower. 



398 The Living Plant 

and tall-growing vines, where birds most frequent; though they 
are by no means absent from low-growing herbs where they are 
eaten by ground birds or some of the smaller mammals. The 
indigestible coatings are formed either by seed coats, as in Grape, 
by ovary walls, as in Strawberry, or by a part of the ovary, as in 
the stone of the Cherry. Sometimes instead of the hard coat, an 
inedible core is developed, which is carried away but not eaten, 
as in Apples, while in yet others the slippery seeds are hardly 
swallowed at all, but are scattered around as the pulp is devoured, 
as in Oranges. The pulp is formed from the most diverse parts, 
one can almost say every possible part, from ovary as in Grape, 
receptacle as in Strawberry, bract as in Juniper, seed coat as in 
Yew, calyx as in Wintergreen, placentae as in Watermelon, or 
hairs as in Oranges. The colors in general are such as are most 
conspicuous under the special conditions prevailing where the 
fruit ripens. Thus red is the most common of the colors of fruits, 
and it is that which is most conspicuous against the green of 
foliage; but purple or blue is more common in fruits of the autumn 
which ripen when the foliage has turned yellow or red, while 
white occurs in some berries which grow in the dusk of shady 
places near the ground. Before they are ripe these fruits are 
commonly sour, or astringent and unpalatable, and, moreover, 
are green in color, precisely like the foliage. This color may serve 
to prevent their notice by animals before the seeds are ripe, al- 
though such a function for the green color is probably wholly 
incidental and secondary to its use as accessory food-making 
tissue. 

A special phase of dissemination by animals, the importance of 
which has only lately been realized, is the transport of seeds of 
low-growing herbs by ants. Such seeds are mostly small and 
light, but are provided with an attached reservoir of food-material 
(called the caruncle), attractive to ants, which carry the seeds to 
various distances from the capsules, leaving them where the food 
has been used. It has also been supposed that some seeds which 




How Plants Secure Change of Location 399 

happen to resemble the larvae of ants are transported some dis- 
tance towards their nests by the same more industrious than 
sensible insects. It is possible, furthermore, that a somewhat 
similar explanation applies to certain seeds or fruits which look 
remarkably like beetles, as do some 
Castor Beans, or like caterpillars, 
as do some members of the Pea 
Family (figure 170) ; for such seeds, 
which are protected by hard coats 
against digestion, are supposed by 
some naturalists to be swallowed 
by birds in the belief that they are 
really live insects. Again, brightly- 
colored hard seeds, protectively coated, appear to be swallowed by 
birds, as are other bright objects, simply because of their attract- 
ive or conspicuous appearance. But these latter matters are doubt- 
ful, and perhaps are fancies rather than facts, though we must re- 
member that strangeness or seeming improbability are not valid 
scientific objections to any explanation of a natural phenomenon. 
Not only birds and small mammals, but also bats, snails, insects, 
fish, and perhaps other animals, have been detected in carrying 
seeds by some one or the other of the various ways we have 
mentioned. The subject is by no means exhausted, and most 
interesting discoveries without doubt still await the keen-sighted 
and persistent observer. 

To complete the subject of transport by animals we must 
mention the action of man, though his agency is of course in- 
cidental and not adaptational. Unintentionally he has spread 
weeds from country to country, until some occur all around the 
world, while deliberately he has carried the plants that are valua- 
able to him to all parts of the earth. Indeed, upon this latter end 
he concentrates much effort and thought, reaching their culmi- 
nation in the deliberate and systematic attempts of our national 
Department of Agriculture to gather useful plants from all parts 



400 The Living Plant 

of the world, and to establish in this country every kind of plant 

which can possibly be of service to our people. 

Before leaving dissemination it is desirable to note certain 

adaptations which are correlated therewith, though hardly a 

part of transport itself. Thus, some seeds 
seem to possess a certain power of plant- 
ing themselves through the movements of 
a definite hygroscopic mechanism which is 
so built as to bore the seeds into the ground ; 
the wild Erodium, and the grass Stipa 
pinnata (figure 171) are examples. In other 
cases the ripening fruit-stalk turns away 
from the light, and thus carries the seeds 
into clefts of the rocks or the cliffs on 
which the plants grow, thus ensuring their 
fall in a place conformable to the habits 

FIG. 171. The fruit of Stipa f ,, , , ,, . . , - , 1 T . 

supposedly self- of the plant; this is true of the Lmana 




lhe n text g : a8 explained in Cymbalaria of Europe, already described 
in another connection (figure 81). Some 
plants place the seed pods in a protective position while ripening. 
This is common in water plants, which draw the fruit under 
water by a spiral coiling of the stem, and in the Peanut, which 
draws it underground. Others, which have seeds scattered by 
wind, greatly elongate the stalks of the seed pods during ripening, 
thus raising them to a position more exposed ; it is thus in the Dan- 
delion. Some seeds become attached firmly to moist ground by 
aid of a mucilaginous substance formed from their coats by con- 
tact with dampness, though the advantage thereof is not clear, 
unless the attachment aids the light weight of the seed in provid- 
ing a resistance permitting the root to be forced more readily into 
the ground. Again, in some pods the seeds will not all germinate 
the same year, even under perfect conditions, but some require a 
year longer than others, thus ensuring the perpetuation of the 
plants even if all the seedlings of one year are destroyed by any 



How Plants Secure Change of Location 401 

calamity. Some seed pods, by action of hygroscopic mechanisms, 
open only when the weather is favorable for the particular mode 
of dissemination of their seeds, whether this requires wetness or 
dryness. And there are yet other disseminational adaptations, 
some real, some accidental, some imaginary, described in the 
works of Kerner and of others already referred to in the foregoing 
pages.* 

When we view as a whole the results attained through these 
modes of dissemination, and note how wide is the spread some 
plants have secured through the world, it becomes plain that in 
the long run the sum total of the accomplishments of the non- 
locomotive plants in this regard, is no whit inferior to that of the 
highly-locomotive animals, if not indeed markedly superior 
thereto. This shows the efficiency of the dissemination methods, 



* Dissemination, dealing with prominent and highly developed adaptations, has 
always been one of the favorite topics of ecological study; and it affords valuable 
material alike for amateur investigation, for student themes, and for popular scientific 
articles in the illustrated magazines. In the hope that the reader may wish to follow 
this subject more deeply than my limits allow, I add here the titles of the principal 
accessible works upon it. The foundation work is Ilildebrand's Die Verbreitungsmittel 
der Pflanzen (Leipzig, 1873), an admirable, but all too brief a treatise, which, un- 
fortunately, has never been translated. There is a wonderfully clear and well- 
illustrated account in Kerner's Natural History of Plants (translated by Oliver, 
New York, Henry Holt & Co., 1895). One of the best synopses, illustrating the 
striking cases, is Beals' excellent little book, Seed Dispersal (Boston, Ginn & Co., 
1898), while much briefer though good are Weed's Seed Travellers (Boston, Ginn & 
Co., 1898) and a chapter in Lubbock's Flowers, Fruits, and Leaves (London, The 
Macmillan Co., 1886). Of articles accessible in magazines the best are Folsom's 
Adaptations of Seeds and Fruits, in Popular Science Monthly, 1893, 218, and es- 
pecially Ridley's Dispersal of Seeds by Birds, in Natural Science, Vol. 8, 1896, 186, 
one of the very best discussions of this subject anywhere in print. And of course, 
there is a host of special papers of all degrees of technicality in the various scientific 
magazines. Considering the attractiveness of the subject, it is very remarkable that 
nobody has yet undertaken to prepare a modern cyclopedic work upon it, something 
comparable with the books we possess for cross pollination; and I commend this 
subject to any ambitious young naturalist among my readers, warning him that the 
task is vast and will take him nearly a lifetime, but assuring him that it offers an 
opportunity for just such a distinctive and useful piece of work as most men find the 
greatest satisfaction in doing. There is not in science any kind of a book that is so 
lasting in .value as this, excepting only the one which presents material wholly new. 



402 The Living Plant 

while illustrating with new force the fact that the race is not al- 
ways to the swift. 

Finally, it is important to note that the ways in which plants 
thus secure transport for their seeds, are closely analogous, if not 
identical, in principle with those by which man supplements his 
own feeble powers of locomotion. He cannot swim far, but he 
can provide suitable appliances to make the winds carry him over 
the broad ocean; he cannot fly at all, but he can place suitable 
appliances in front of certain explosive forces and make these 
drive him triumphantly through the air. The difference between 
the Dandelion plume and the ship's sail, or between the explosive 
capsules of Witch Hazel and the aeroplane engine, is merely one 
of detail and degree. Man and plant are doing the same thing in 
essentially the same way, the chief difference being that man 
knows what he is doing while the plant does not. The belief that 
seed-wing and ship-sail have been developed in ways that are 
fundamentally the same helps to explain what I meant in the 
chapter on Protoplasm when I said that all protoplasm can think. 



CHAPTER XVI 

THE METHOD OF ORIGIN OF NEW SPECIES AND 
STRUCTURES, AND THE CAUSES OF THEIR FITNESS 
TO THE PLACES THEY LIVE IN 

Evolution and Adaptation 




P the various aspects which Nature presents to the 
intellect of man, there are two of particular promi- 
nence, facts and explanations. Of these the greatest 
by far are facts, naked, stark, primitive, elemental, 
cosmical facts. They are the raw material of science, and nothing 
can replace them. But when one has made himself master of a 
goodly number thereof, and has arranged them in some kind of 
preliminary classification, he soon comes to crave explanation of 
the remarkable relations they are sure to exhibit. Explanation 
is the office of Philosophy, and there is a Philosophy of Nature. 
The phases thereof most important to the student of animals and 
plants concern the origins of their multifarious kinds, of their 
elaborate structures, and of their remarkable fitness to their 
surroundings. There was a time when none of these were; now 
they all are; when and how, in the interval have they arisen? 
This is the great present problem of philosophical biology, and 
one which the reader, fresh from his contemplation of the facts 
and relationships set forth in the preceding pages, is now pre- 
pared, and I hope eager, to attack. 

There are two great explanations, logically and historically, of 
the origin of species, structures, and adaptations, viz., Special 
Creation and Evolution. The doctrine of Special Creation, held 
almost universally down to a half century ago, maintained that 

403 



404 The Living Plant 

every kind of plant and of animal, with every one of its manifold 
parts, was created substantially as it now exists at some definite 
time in the past by the act of an omnipotent and omniscient 
Creator. On the other hand, the doctrine of Evolution, now held 
by all biologists and most other thinkers as well, maintains that 
each species of plant, and each one of its structures, has been 
derived by gradual modification from preexistent and simpler 
kinds, which in turn were derived from yet other and still simpler 
kinds, and so on, in an unbroken chain of descent back to very 
ancient and very simply-organized ancestors, whose exact mode 
of origin is still quite unknown. 

It is now a long time since it was thought needful to present in 
biological courses or books the evidence for Evolution against 
Special Creation, but our present-day acceptance of Evolution as 
almost an axiomatic truth involves some danger of leaving our 
learners in ignorance of the nature and force of the evidence 
which has compelled its acceptance. I would dearly like to 
present this evidence to the reader as I do to my students, but 
the callous incompressibility of paper and type forbid; and it 
must suffice to say that it is drawn from these several sources : 
from the analogy of plant and animal improvement by man (soon to 
be considered in a separate chapter on Plant Breeding), whereby 
from simple wild forms of both animals and plants, new kinds, 
most diverse and most wonderful, have been produced; from the 
results of classification, which show that the kinds of plants and 
animals fall naturally into an arrangement similar to that estab- 
lished by relationship based upon descent among mankind, some 
of the very same terms indeed (race, tribe, family) being used in 
both cases; from morphology, which shows that the diverse forms 
of special structures, spines, tendrils, pitchers and so forth, 
are all modifications of simpler preexistent structures, usually 
leaves, stems or roots; from the existence of gradations, all the 
way up in regular steps from the very simplest kinds of plants to 
the most complicated, with no notable gaps or missing links in 



Method of Origin of New Species and Structures 405 

the series; from fossils, those relics of ancient plants converted to 
stone and preserved in the rocks, which show that the earliest 
plants to flourish in the earth's history were the simpler kinds, 
while those which came later were progressively more complex, 
and the very highest of all appeared last; from geographical dis- 
tribution, which is such that in general the kinds of plants most 
closely related are found nearest together, while those which are 
farthest apart are most distantly connected; from the existence of 
rudimentary structures, such as the imperfect stamens in irregular 
flowers, or the appendix in man, which are useless to their present 
possessors, but are useful to the near relatives, and hence pre- 
sumably to the ancestors, of the kinds; from embryology, or the 
course of development of the individual from the egg, which 
often exhibits some temporary stages quite useless to the develop- 
ing individual but useful in those ancestors which the form must 
have had if evolution is a fact; and from yet other sources which 
need not here be particularized. In all of these directions the 
phenomena are perfectly explained by evolution, but present well- 
nigh insuperable logical difficulties to an explanation by special 
creation. Or, the case can be stated in this way, if evolution be 
assumed, then the facts are intelligible, but if special creation be as- 
sumed, then they are enshrouded with inconsistency and mystery. 

I may venture at this point to remind the reader, though 
probably the caution is needless, that the question as to whether 
evolution is or is not a fact is a purely scientific one, to be judged 
by purely scientific evidence, tested by inexorable scientific logic. 
It is fatal to a correct judgment upon such a subject to approach 
it with preconceptions or prejudices of any kind, metaphysical, 
personal, or religious; for the mind of man is so organized that 
whenever it seeks evidence for some favorite belief, it has no 
trouble at all to find it. To him who puts on colored glasses, all 
things look of that color; but evolution is something to be viewed 
only in the purest white light of the truth. 

But while evolution is accepted as a fact by the concensus of 



406 The Living Plant 

present biological opinion, biologists differ much in their opinions 
as to the method by which it has been effected, for of course the 
fact or non-fact of evolution is one thing, and the method whereby 
it has been brought about is another. Evolution may be true 
and yet every one of the explanations of the method thereof, 
given heretofore by scientific men, may be false. These explana- 
tions, however, are so important in many ways that we must now 
proceed to consider them. 

Of all the explanations of the method of evolution, the greatest 
and best known is Darwin's, embodied in his principle of Natural 
Selection. It was, indeed, the first logically-satisfactory explana- 
tion ever given of the way in which evolution may have been 
brought about; and, because it was logical, it enabled thoughtful 
men for the first time to believe in evolution as a fact, for they 
could not believe in its reality so long as they could not under- 
stand how it might have been effected. Herein consists Darwin's 
greatest service to science; and this, moreover, is the reason why 
his name is associated with evolution so closely that most people 
regard the two words as practically synonymous. And the case 
is not at all affected by the fact that Natural Selection may yet 
prove not to be the real explanation of evolution. It is a possible 
and a logically-adequate explanation, but not necessarily on that 
account the correct one. 

So important is this principle of Natural Selection, historically 
as well as scientifically, that we must now consider it sufficiently 
to make its significance clear; and this is the more needful be- 
cause it is commonly misunderstood even by many of those who 
talk much about it. I shall try first to present the subject as 
I think that Darwin conceived it, giving later the modifications 
introduced by subsequent investigation. 

In essence Natural Selection is a deduction from the inter- 
operation of five factors, all of which are familiar to observation, 
viz., variation, overproduction, struggle for existence, survival of 
the fittest, heredity. 



Method of Origin of New Species and Structures 407 

Variation. It is a matter of familiar knowledge that all 
living things, or the structures they produce, even the most 
closely-related, are different from one another, to such a degree, 
indeed, as to justify the common saying that no living thing 
is exactly like any other living thing. The differences, or vari- 
ations, affect every possible feature, size, form, color, texture, 
etc., and occur in every possible direction; and some of them 
at least are inherited from the parents and transmissible to 
offspring. 

Over-production. All living beings possess a power of repro- 
duction not only sufficient to replace the individuals which die, 
but also to increase greatly their numbers. Moreover, the rate of 
increase, in even the slowest breeding forms, is surprisingly rapid, 
while with most kinds it is enormously so. Thus, a plant which 
produces only ten seeds a year (and few produce so very small a 
number), would have one billion descendants within ten years, 
and would soon cover the earth to the exclusion of all others could 
its increase proceed without hindrance. 

Struggle for Existence. Although every kind of plant and of 
animal is thus tending to increase enormously in numbers, never- 
theless in a broad way those numbers remain stationary from one 
generation to another. Local fluctuations do of course occur, for 
some kinds of plants or animals are on the way to extinction, 
while others, such as weeds or insect pests, have periods of rapid 
expansion; but in general it is true that there are no more of any 
particular kinds, lichens, goldenrods, thrushes or squirrels, in 
a given region one year than another. The reason thereof is 
obvious enough, the world is already as full of animals and 
plants as there is food or room for, and new ones can find a place 
only as the old ones die out. This, then, is the situation; that 
while great numbers of plants and animals are born into the world 
in each generation, there is only room or food for an occasional 
one of the number. But as each and every one of the individuals 
thus born has an equal right and impulse to survive and possess 



4o8 The Living Plant 

itself of the scanty room and food, there results among them a 
constant struggle for existence. 

Survival of the Fittest. In this struggle for existence among a 
great many individuals of which few can survive, what determines 
which those few are to be? If the young individuals were born 
all alike the survival would obviously be determined by nothing 
but chance; but in fact they are all born unlike, and among the 
differences, or variations, which they exhibit, there must happen 
to be some which fit their possessors better for the conditions of 
the particular struggle in hand than do others; and such better- 
fitted forms will naturally be the ones to succeed. This is the 
survival of the fittest. Where the seedlings of spruce trees spring 
up of themselves in fields that are abandoned, it comes finally to 
pass that a few tower upward in full vigor, while the shade under- 
neath them is almost like night from the profusion of dead stems 
of the unsuccessful, the ones which did not possess the variation 
of most rapid upward growth to possession of the indispensable 
light. 

Heredity. In reproduction, as everybody knows, the main 
features of the parents are repeated in their offspring, or are 
hereditary. Now this is true also of at least a part of the varia- 
tions. Hence, when a plant or an animal survives by virtue of 
some particular advantageous variation, that variation is likely 
to be repeated in its offspring. Meantime, of course, the unfit 
have perished, and left no descendants. The whole tendency, 
therefore, is towards the production of a race in which the valuable 
variation is universal. 

It will now be evident to the reader that these five factors acting 
together must tend to cause a natural selection, and hereditary 
fixation in each generation, of the fittest, or most advantageous 
variations, of whatsoever kind. It remains to consider, and this 
is a point too often overlooked, how a variation can accumulate 
and become intensified, generation after generation, until it forms 
a well-marked character of the species. This, also, is easily under- 



Method of Origin of New Species and Structures 409 

stood on reflection. For not only do the offspring of the parents 
preserved by possessing a fit variation inherit that variation, but 
they vary in regard to that variation itself. Therefore in any 
generation, while some of the individuals will inherit the variation 
about like the parents, a few will vary towards a greater intensity 
thereof; and in the struggle for existence in this generation, 
these more extreme individuals will survive. Their offspring, 
in turn, will tend to resemble them in possessing the greater 
degree of the variation, but the offspring will include some that 
vary towards an even higher degree, and these will survive, and 
so on. Thus a variation, by its continued selection in one direc- 
tion generation after generation, can pile up until it produces a 
large and visible change in that feature of the plant. But the 
different features of the organism are so closely tied together that 
a change in any one always involves some others, while, moreover, 
selection may be operating upon more than one feature of a plant 
at a time. Thus the accumulation of variations gradually make 
their possessor look distinctly different from the original ancestors; 
and when that point is reached we call it a new species, especially 
if, as usually but not invariably happens, the intermediate and 
less well adapted forms have died out in competition with the 
better. The process is represented in operation, with increase 
in size assumed as the advantageous variation, in the accom- 
panying diagram (figure 172). This process of progressive adapta- 
tion would continue until the species theoretically has become as 
perfectly adapted as possible to the selective conditions; but in 
fact such stability would never be reached since the conditions 
themselves, like all the rest of the world, are in continual altera- 
tion. And such is THE ORIGIN OF SPECIES BY MEANS OF NATURAL 

SELECTION, OR THE PRESERVATION OF FAVORED RACES IN THE 

STRUGGLE FOR LIFE, in the words of the title-page of Darwin's 
greatest book. 

The theory of Natural Selection explains very perfectly not 
only the origin of new structures and new species, but also the 



The Living Plant 



FIG. 172. A diagram to illustrate the three leading 
explanations of the method of evolution. 

A spherical organism, represented by the circles, is 
assumed to be living under conditions where in- 
crease of size is advantageous. The course of 
evolution is condensed to eleven generations, be- 
ginning on the left and running up to the right. 
The dotted lines show the connection between 
parents and offspring, and the shading in- 
dicates extermination of the less fit. 

The upper figure represents the operation of 
natural selection. Two offspring are as- 
sumed in each generation of which 
one varies to a size larger than 
the parent, the other remaining 
the same. 

The second figure represents 
the operation of transmis- 
sion of acquired char- /~~\"- : 
actors. As all the /( V* fe| 



offspring arc 
from this 
point of 
view, only 
one of each 
generation 



alike 





Q'QDJ 



is shown. The 
adult individuals 
are assumed to ac- 
quire a larger size 
under stimulation, 
and to transmit 
that larger size to 
their offspring. 
The lower figure rep- 
resents the operation of mutation. The indi- 
viduals are substantially alike for a number 
of generations, then suddenly give origin to 
a larger type, which persists unchanged for 
a time, only to give origin to a still larger, and 
so on. 




cause of their adaptations to their surroundings. It will be evident 
to the reader that the principle, while logically strong, is highly 



Method of Origin of New Species and Structures 411 

hypothetical; and, needless to say, mankind has not yet seen the 
natural evolution of a species by this process. It has a weakness 
in the fact that all of its reasoning is on the assumption " other 
things being equal/' whereas in fact the innumerable other things 
rarely are equal. It has a strength, on the other hand, in the fact 
that the kind of artificial evolution effected by man in the produc- 
tion of new kinds of animals and plants, uses precisely and solely 
the same method of selection and preservation of variations. 
There is, however, this notable difference between the products 
of artificial and natural selection, that the former tend always to 
revert back towards their former condition, while apparently the 
latter do not; and to many observers this difference seems fatal to 
any support of natural by artificial evolution. It may be, how- 
ever, that the time element in the process is important, and that 
the comparative rapidity with which man makes his new kinds 
does not allow the new characters enough time to "set." If one 
keeps a band of rubber stretched only a brief time it springs 
back to its old shape; if longer, only partly; if long enough, not 
at all! 

It is difficult for anyone, and impossible for me, to think at 
much length about Natural Selection without recalling its great 
author. Science hath her heroes no less than war, and Darwin was 
one of our noblest. An Englishman, born in 1809 to singular good 
fortune in material things, and fortunate in the influences which 
molded his intellectual life, he came slowly to his great concep- 
tion, which he first published when he was fifty years old. This 
was in his book The Origin of Species, which by common consent 
is agreed to have exercised a more profound effect than any 
other secular book upon human thought. It is difficult for us 
in these more liberal days to comprehend the bitterness of the 
opposition which his support of evolution aroused, partly among 
the older naturalists but chiefly among those who imagined that 
the foundations of religion were endangered. But through all the 
storm he stood steadfast, calm, just, and magnanimous, even 



4i2 The Living Plant 

though to his other great provocations was added the torment of 
chronic ill-health. Of him his friend Huxley has said, "The 
more one knew of him, the more he seemed the incorporated ideal 
of a man of science." Possessing vast speculative powers, he 
nevertheless kept his imagination in touch with the truth by in- 
cessant and laborious observation and experiment. Yet this 
greatest of all naturalists was no demi-god, much less a person 
abnormal to his kind, but a warm-hearted, humanly-interested, 
honorable-souled gentleman. He lived to see the complete 
triumph of his life-work, and died in high honor in 1882 at the age 
of seventy-three.* 

The second in importance, though first in time, of the great 
explanations of evolution was Lamarck's principle of the " trans- 
mission of acquired characters." It is almost the exact logical 
opposite of natural selection, and the life of its author contrasts 
almost as greatly with that of Darwin. A Frenchman, born in 
1744, he was at the height of his career about fifty years before 
Darwin, as Darwin was fifty years before our own time; and it is 
a coincidence of no little interest that the work in which he most 
fully expounded his views was published in 1809, exactly fifty 
years before the Origin of Species. But Lamarck, unlike Darwin, 
failed to keep his imagination checked by investigation, and his 
theories in close touch with the facts. Therefore he had the 
mortification to see his favorite work ignored by his contempo- 
raries; and he died, in 1829, in disappointment, infirmity and 

* The reader will wish to know more about Darwin, and will find great satisfaction 
in a study of his Life and Letters (one of the great biographies of literature) by his 
son Francis. In that work, his own autobiography, and his son's reminiscences, are 
of first interest, but the most charming glimpses of his character are given by his 
letters, for example, that written to his wife from Moor Park, in April, 1858, and 
that written to his friend Asa Gray, on August 9, 1862. And the reader should not 
fail to read the remarkable obituary of Darwin by Huxley in Nature for April 27, 
1882, doubtless the noblest tribute ever paid by one scientific man to another. 
The Origin of Species is not an easy book to read, nor can it be really appreciated 
by anyone until he has acquired a considerable background in biological knowledge: 
but after that the reasons for its real greatness become clearly apparent. 



Method of Origin of New Species and Structures 413 

poverty. Yet though his labors were seemingly without immedi- 
ate fruit, they were of great service, nevertheless, in awakening 
men's minds to the problems and possibilities of evolution, and 
thereby making the way easier for Darwin. 

Lamarck's theory is founded on two factors, the alterability 
of individuals, and the hereditary transmission of the results 
thereof. 

The Alterability of Individuals. This is not theory, but fa- 
miliar fact. Everybody knows that our own muscles, with their 
associated blood vessels, nerves, and bones, can be improved in 
size and strength by exercise, as can our minds, and other features 
of our being; and likewise these can all degenerate through disuse. 
And so it is throughout the animal kingdom. The process is well 
understood: the use of the part serves as a stimulus which leads 
the organism to throw more material and energy into those parts, 
precisely in the manner which we have studied already in the 
chapter on Irritability. In the same chapter we have seen also 
how plants can alter their structure under stimulation. Thus 
some kinds of plants can develop a thicker epidermis under the 
stimulus of dry air; some trees can apparently build stronger 
stems under stimulation of bending by the wind ; and indeed there 
seems to be no limit to the directions and degrees in which plants 
can respond structurally, and adjustively, to stimulation. The 
structural alterations thus produced in individuals, are called 
acquired characters. 

Transmission of Acquired Characters. This is the crucial point 
in the theory of Lamarck. He held that characters acquired 
during the life of the individual, as described above, are 
transmitted to their later offspring, which, therefore, exhibit 
larger muscles, or finer minds, or thicker epidermis, or stronger 
stems, than would have been the case had the parents not devel- 
oped those features. Lamarck actually uses the illustration of 
the blacksmith's arm, powerfully developed in the practice of his 
trade; and he maintains that the later-born sons of the smith will 



414 The Living Plant 

have more powerful arms than would have been the case had 
their father adopted some less strenuous trade. Most of our 
popular beliefs tend to the same end, especially as to moral and 
intellectual qualities; for it is commonly supposed that the finer 
mind developed in an individual by high education, or the de- 
generacy produced by submission to vice, are somehow trans- 
mitted to the offspring. If a feature is hereditary for one genera- 
tion, however, it is hereditary for more; and thus, according to 
Lamarck, a character can go on piling up generation after genera- 
tion until it reaches a degree of development sufficient, along with 
associated changes, to make its possessor rank as a new species. 
Of course, on this principle, all individuals born into the world 
have an equal chance for survival, and mere chance would de- 
termine success. This method of evolution is illustrated in com- 
parison with that by natural selection on the accompanying 
diagram (figure 172). 

The Lamarckian explanation of evolution has a great merit in 
its simplicity, but has the fatal defect that the crucial trans- 
mission of acquired characters is not confirmed either by ordinary 
observation, or by any experiment which has been devised to 
test it. Moreover, that gigantic system of experiment always in 
progress in plant and animal improvement by man has failed to 
yield one fact in its support. Furthermore, the phenomena 
which apparently are the strongest in its favor can be explained 
more simply in other ways. Thus, the blacksmith's sons, it is 
true, tend to have stronger arms than ordinary men; but this 
need not mean that they inherited the stronger arm acquired by 
the father, but only that they inherited the same robust proto- 
plasm which enabled the father to become a successful smith. So 
the children of highly educated parents are apt to be bright, not 
because they inherited the educated minds of the parents, but 
because they inherited the finer quality of mind-protoplasm 
which made high education in the parents a possibility; and so 
with the children of tuberculous parents, who inherit not the 



Method of Origin of New Species and Structures 415 

tuberculosis, but the weak lungs which render tuberculosis pos- 
sible. Taken as a whole, therefore, the evidence we possess upon 
the subject does not tend to support the Lamarckian theory. 

The contrast between the theories of Darwin and of Lamarck 
is given the sharpest definition by the work of Weismann, an 
eminent German zoologist still living. Darwin himself, while 
convinced that natural selection was the leading factor in effect- 
ing evolution, was inclined, especially in later life, to admit some 
transmission of acquired characters; and indeed he actually in- 
vented a special theory (called pangenesis, a flow of tiny solid 
particles from all parts of the body to the germ cells), to explain 
a possible mode of its operation; but his follower Weismann stood 
for Natural Selection, pure and simple, as against the rival theory, 
and even invented an ingenious conception to explain the natural- 
ness of the operation of the one and the impossibility of the other. 
In brief, he held that there are two kinds of protoplasm in each 
animal, one reproductive, the germ plasm, confined to the eggs 
and the sperm cells, and the other the body plasm, making up all 
the rest of the organism. Now the fertilized egg-cell, from which 
the new individual grows, is obviously germ plasm. As it grows 
.and develops, a part of the resultant cells keep on being germ 
plasm, which, however, remains latent until the animal is adult, 
while the remainder of the cells develop into body plasm, which 
grows immensely and comes ultimately to surround, protect and 
nourish the embedded germ plasm. Then when the time for 
reproduction has arrived, it is always the latent germ plasm, 
never the body plasm, which builds the new egg-cells and sperm 
cells, whose union starts another individual in the same way as 
before. Thus germ plasm produces body plasm, but body plasm 
never produces germ plasm. Hence the germ plasm is potentially 
immortal, keeping on as one continuous line of tissue from genera- 
tion to generation, while the body plasm is mortal, made anew in 
each generation and perishing utterly therewith. The matter can 
be expressed also in this manner, that the germ plasm forms a 



4i 6 The Living Plant 

kind of continuous axial thread upon which the body plasm is 
strung at intervals like beads on a string, except that we have to 
imagine the beads as growths from the string! On this theory 
the germ plasm is the essential protoplasmic basis of the race, and 
the body is simply an organ which it builds to secure its own 
nutrition and protection. Now it is obvious that any variation 
which originates in the germ plasm can show itself in all of the 
succeeding germ plasm, and also in all of the bodies which grow 
out therefrom; on the contrary, any variation, or other feature, 
including an acquired character, which originates in the body 
plasm, must perish with that body and cannot affect the bodies 
which come after, unless it can go round through the germ plasm, 
for which no mechanism is known to exist, excepting possibly 
that mentioned in a paragraph to follow. This theory of Weis- 
mann's explains very perfectly an evolution by natural selection 
of innate (in-born or germ-plasmic) variations, and also supplies 
a reason for the non-transmissibility of acquired characters. It 
shows how children can exhibit cultured minds or large muscles 
like their parents without inheriting the results of their parents' 
culture or exercise, for while the results of culture or exercise are 
confined to the body plasm, and perish when it does, the capacity 
to develop the results depends on the constitution of the germ 
plasm which parents and children share alike. The bodies of 
children resemble the bodies of their parents, therefore, not be- 
cause the former are derived from the latter, but because both 
are derived from the same source. This ingenious theory of 
Weismann's has not been confirmed by further research so far as 
its physical basis in the two kinds of protoplasm is concerned, and 
it never applied well to plants, which seem very clearly at times 
to create germ plasm out of body plasm; but I give it this much of 
our attention because all recent research is tending to confirm the 
correctness of its central principle, which stands perfectly when 
expressed in this way, that body characters are derived from 
germinal determinants, which in turn are derived from preceding 



Method of Origin of New Species and Structures 417 

germinal determinants in a continuous line, but never from body 
characters. And the fallacious physical basis given his theory by 
Weismann has been replaced by a secure one supplied by Mendel- 
ian studies, presently to be considered. Indeed, the modern con- 
ceptions of heredity, based on Mendelian results, is a veritable 
reincarnation of the central feature of Weismannism. 

Before leaving this part of the subject it is needful to say that 
such evidence as does seem to favor a transmission of acquired 
characters is found in connection with certain diseases. The cases 
seem to hinge upon chemical changes produced in the blood, which, 
circulating and diffusing throughout the whole body, can thus 
reach the germ cells and through them the next generation. On 
this basis, any acquired character which affects the chemistry of 
the blood could, theoretically, be transmitted to the germ plasm 
and the next generation, although changes which are simply of a 
physical or mechanical nature could not. We have a close anal- 
ogy in the relation of scion characters to stock characters in 
grafting, already considered (page 350); for it appears to be 
generally true that characters which are dependent upon the sap 
can be extended or "transmitted" from the scion to the stock, and 
vice versa, while characters which are dependent upon the pro- 
toplasm are confined, on the contrary, strictly to scion or stock 
respectively. This principle of chemical transmission may yet 
prove to be important in evolution, and may rehabilitate Darwin's 
theory of pangenesis on a new basis; and it accords with the tend- 
ency of all modern research to reduce natural phenomena to a 
chemical foundation. Indeed, some students of the subject have 
suggested that the chromosomes, those carriers of heredity in the 
nuclei of cells, are simply collections of enzymes, each of which 
controls some single process of development in the new individual. 

The study of the problems of evolution exhibits three separate 
epochs. The first was that of speculation from impressions, cul- 
minating in the theories of Lamarck. The second was that of 
induction from observation, inaugurated and carried to highest 



4i 8 The Living Plant 

perfection by Darwin. The third is that of test through experiment, 
of which we are witnessing the very beginning. Its great leader 
is de Vries, an eminent Hollander still active in scientific service. 
Some twenty years ago de Vries noticed in Holland a certain 
weed, an American Evening Primrose, called CEnothera Lamarck- 
iana (note the coincidence of name!), which showed such re- 
markable phenomena of variation that he brought some of the 
plants into his botanical garden where he could study their be- 
havior with exactness. The result was remarkable indeed, for 
he saw new kinds originating before his eyes, not by any slow 
process, but the fastest that is physically possible, viz., in 
one step from parent to offspring. When seeds were taken from 
the ripe pods of CEnothera Lamarckiana, and planted with pre- 
cautions which precluded all possibility of error, most of the seeds 
grew into plants like the parents; but some grew into a much 
smaller kind, others into a much larger kind, and yet others into 
other kinds, differing in other respects (figure 173). Thus from 
the parent species several daughter kinds were produced, and not 
once alone but regularly generation after generation. The new 
kinds do not differ much from the parent, but enough to enable 
trained botanists to distinguish them with certainty; and more- 
over they differ not in one but in a great many features. The 
individuals of any one of these new kinds exhibit minor, or fluc- 
tuating, variations among themselves it is true; but they preserve 
throughout a sufficiency of definite characters in common. Fur- 
thermore, and this is a matter of the very greatest importance, 
when seeds of each of the daughter kinds were planted by them- 
selves, they reproduced each their own kind, and that not alone 
for one generation, but for several, and indeed for as many as 
time has allowed since their discovery. Finally, the same ex- 
periments have been repeated elsewhere, and with identical re- 
sults. There seems no doubt, therefore, that this species of 
Evening Primrose is actually giving off several new kinds year 
after year, and kinds which reproduce themselves permanently. 



Method of Origin of New Species and Structures 419 

Such new kinds were supposed by de Vries to be species; of an 
ultimate or elementary sort, and, in reference to thei? anode of 
origin he designated them mutants, while the parent species he 
described as being in mutation. As to the exact relation of muta- 
tion to evolution, that was supposed by de Vries* to be this, that 
mutants, or elementary species, and not single variations, are 




Fia. 173. Groups of the mutants of (Enothera, growing in de Vries' experimental garden 
at Amsterdam. The parent species, (E. Lamarckiana, is the single one on the extreme 
left. In one group two flowers are covered with bags for experimental purposes. Ob- 
serve the distinctness of the groups from one another, in conjunction with a certain 
amount of variability within each group. (Photographed from a colored picture in 
de Vries' book, The Mutation Theory, Vol. II.} 



the material upon which selection works. Darwin thought the 
basal variations were mostly single, finely-graded, and more or 
less unstable, while de Vries offers instead collections of varia- 
tions large, definite and permanent; but otherwise their views are 
in full harmony, both agreeing that natural selection is the final 
factor which determines survival. Like variations, the mutations 



420 The Living Plant 

which happen to be adaptive would be preserved in the struggle 
for existence, while the unadapted would perish. Then, according 
to the theory, after a time the surviving species would mutate 
again, and the fittest of its mutants would survive, and so on. 
Thus would the species be kept adapted approximately to en- 
vironments, while evolution would take place in a series of short 
abrupt steps separated by long pauses, a condition illustrated 
in the lower part of our diagram (figure 172). De Vries' view of 
evolution has, moreover, one marked advantage over Darwin's 
in explaining the existence of species and structures which bear 
no adaptive relations to the environment, such for example as 
the wonderfully diverse forms and markings of the Diatoms; for 
such features can originate and reach a considerable degree of 
development by mutation, if not cut off by natural selection, 
while on Darwin's view only those things which were adaptive 
had any chance of a considerable development. 

The great interest of de Vries' work at once stimulated a wide 
search for other examples of mutation in both animals and plants; 
and a very few other cases have been discovered. Attempts have 
also been made to set species into mutation experimentally, but 
with only dubious success. It is thus plain that species in muta- 
tion are very few, which fact is explained by de Vries on the sup- 
position that species display short periods of mutation separated 
by long periods of quiescence. 

But though species in mutation are undoubtedly rare, species 
apparently identical with mutants have been found to be common, 
though there is no evidence as to how they have arisen. The more 
intensive studies of the past few years have shown that species 
formerly considered single are in reality aggregates of dozens or 
hundreds of these mutant-like species, which have now become 
so prominent in scientific literature that they have attained to 
a distinctive designation of their own, viz., elementary species or 
biotypes. Thus the little "Ladies Tobacco" of our earliest spring 
fields, thought by the acutest observers of the last generation to 



Method of Origin of New Species and Structures 421 

represent but one species (viz., Antennaria plantaginifolia) , has 
been found to include a dozen, all perfectly distinct, permanent, 
and recognizable by good observation; the Brambles and some 
Grasses have been claimed to include not dozens of species but 
hundreds; and the Hawthorns of America have already been 
described to the number of over a thousand, with no end to the 
trouble as yet. The same thing is true also of cultivated plants, 
and strikingly so of the grains. A field of Indian Corn, for ex- 
ample, has been found to consist not of one species, as we used to 
suppose, but of dozens of biotypes, or elementary species, crossing 
and hybridizing greatly it is true, but capable of separation and 
ultimate pure breeding each by itself. It is important to remem- 
ber, however, that the fact of the existence of these elementary 
species is quite independent of the question as to their origin; and 
many of those who have had most to do with their discovery 
doubt whether they have arisen by mutation, though de Vries, of 
course, believes that they did. 

There is one other great name associated with evolution, even 
though somewhat indirectly, arid that is Mendel, whose dis- 
coveries in the particular field of heredity are exerting a profound 
influence upon present-day evolutionary thought. We have 
already discussed his work in our chapter on Reproduction, and 
need only summarize here the points of importance to our im- 
mediate subject. They are these: 

First, each individual organism, animal or plant, is an aggre- 
gate, or mosaic, as it were, of a definite number of characters 
each of which is represented by a determiner or unit in the germ 
cells from which it has developed. These characters are thou- 
sands in number in the higher organisms, fewer in the simpler, and 
include all kinds of features of structure, form, size, color, etc. 
Thus eye-color in man, and the number of rows of grains on an 
ear in Corn, are such unit characters. 

Second, every germ cell, whether egg-cell or sperm-cell, con- 
tains one complete set of units capable of reproducing all the 



422 The Living Plant 

characters of the organism, but never any duplicate units. On 
the other hand, the fertilized egg-cell, and every cell of the body 
subsequently arising therefrom, contains a duplicate or double 
set, one from each parent, from which selection has to be made 
during the development of the organism; and this double set pre- 
vails until the new formation of the germ cells, each of which 
in the "reduction division" receives but a single set. 

Third, while each one of these newly-formed germ cells contains 
a complete set of units, these are partly derived from one of the 
parents, and partly from the other. Moreover, in any given germ 
cell, the paternal and maternal units are mixed in the most com- 
plicated manner, and, furthermore, hardly any two germ cells can 
be found with the same combination. Consequently when the 
unions of these germ cells in reproduction are left wholly to 
chance, as Mendel's results prove that they are, then the most 
diverse possible combinations of paternal and maternal charac- 
ters must result, even among close-fertilized kinds such as many 
plants are, while the complications are proportionally greater 
among cross-fertilized beings, like mankind. We have thus the 
explanation of the very familiar fact that no brothers or sisters 
are ever found who exhibit the same combination of characters of 
father and mother, even the case of identical twins being no real 
exception, since these are known to arise from the splitting of one 
fertilized egg-cell. 

We have now brought our subject quite down to our own days, 
which are distinguished by extreme activity in experiment. It is 
not possible, however, to estimate as yet the value of the results 
that seem to be accruing therefrom. We lack perspective, of 
course; and moreover the conclusions have not yet received the 
thorough critical testing which is essential to establish that 
"impersonal validity," without which they cannot rank as 
scientific knowledge. But I shall add here a synopsis of the 
principal matters which seem to be crystallizing out from these 
studies. 



Method of Origin of New Species and Structures 423 

First, exact studies on variation seem to show that the fortu- 
itous variations of Darwin are of two distinct kinds or classes. 
One class, now called fluctuating variations, includes those caused 
by the immediate environment acting either forcibly (prodiicing 
injury, &c.), or through stimuli calling out irritable responses; 
they are not hereditary, and therefore have no influence in evolu- 
tion. They are variations of the body plasm only, in Weismann's 
sense. The reader will find good examples of variations of this 
type in the differences between the individuals within each of the 
mutation groups shown in figure 173. The other class includes 
those that are inborn, and hereditarily transmitted from genera- 
tion to generation, variations of the germ plasm, in Weismann's 
sense. These are variations upon which natural selection works. 
Their origin is unknown, but they are related if not identical with 
mutations, and with permutations and combinations of Mendelian 
unit characters. The two classes of course, are indistinguishable 
by the eye, and only determinable by experiment. 

Second, the very newest studies, announced during the writing 
of this book, appear to be demonstrating that the mutants of 
Evening Primrose discovered by de Vries, are simply in large part 
the separation or segregation out of original elementary species 
which hybridized together to form the original (Enothera La- 
marckiana. This case of mutation, therefore, is not an instance 
of the appearance of new species, but simply of the reappearance 
of old ones temporarily obscured in a combination; and it leaves 
unsolved the question of the origin of elementary species. 

Third, all the recent work is confirming the reality of the exist- 
ence of elementary species or biotypes, though it is throwing very 
little light on their origin. Moreover, and here is a most important 
point, it is showing that these biotypes, though apparently homo- 
geneous (and therefore forming a single phaenotype), are in reality 
composite, since they embrace a good many Mendelian com- 
binations (or genotypes). But it is not worth while to follow these 
matters further at present, since we now verge close to the firing 



424 The Living Plant 

line, where issues are doubtful. It must suffice to say that our 
knowledge of these subjects is in process of active extension at 
this moment. 

Fourth, exact study devoted to determining whether the selec- 
tion of variations, in the Darwinian sense, can actually produce 
a new species have given very largely a negative result, much 
evidence tending to show that selection simply isolates the bio- 
types, but cannot in any way alter them. If, however, biotypes 
originate from other biotypes, as it seems that surely they must, 
then the method of evolution would be substantially that im- 
agined by de Vries for his mutants, and that represented in our 
comparative diagram (figure 172). Thus, selection would still 
rank as the great decisive, though not as an originating, factor 
in evolution. As to adaptation, that still stands as a corollary of 
any kind of evolution by selection, for selection imposes a step- 
by-step development in touch with the environment. The con- 
ception of biotypes is wholly consistent therewith, and indeed 
helps to explain some of the peculiarities of adaptation, es- 
pecially the somewhat loose, clumsy, or generic character that 
most adaptation displays in conjunction with the occasional 
existence of highly exact fitness. In general in Nature, structure 
fits function about as well as a man's physique fits his trade, 
that is, always in a general way, and sometimes very exactly. 
We cannot expect rigid biotypes to fit intergrading environments 
any more than we can expect polygons to match circles, though 
with some many-sided kinds, the correspondence can be appre- 
ciably close. But it is perfectly clear that the first great problem 
of present-day experimental evolution is the determination of the 
origin of biotypes, or, to be exact, of the variations or characters 
which constitute biotypes. I should not be surprised if it were to 
turn out that the origination of new characters or biotypes is a 
normal function of organisms, adaptively acquired by them pre- 
cisely as any other physiological function has been, and represents 
their method of securing survival in changing environments. It 



Method of Origin of New Species and Structures 425 

is a voluntary offering of material, so to speak, for selection to 
choose from. 

Such is the present state of uncertainty in our knowledge of 
some of the most fundamental matters in evolution. The truth 
we shall learn later through intensive study and experiment. At 
many places the world over, at the Desert Botanical Laboratory 
in Arizona, at the Station for Experimental Evolution on Long 
Island, at many Government and University Stations in Ger- 
many, England, and this country, trained experts, under the 
best of conditions, are subjecting these problems to the test of 
rigid experiment. The results are sure to be important and may 
be revelational. It is one of the great privileges of living in this 
age that we may witness these advances, and may even have 
part in them. There is in store for us all, who are students of 
biological science, many a thrill of purest delight as we open the 
pages of our weekly scientific newspaper, our Nature or our 
Science, and find the first announcement of discoveries which 
will later illuminate one by one the dark problems of nature. 
Science has indeed good reason for her distinctive optimism. 



CHAPTER XVII 

THE REMARKABLE IMPROVEMENT MADE IN PLANTS 
BY MAN, AND THE WAY HE BRINGS IT ABOUT 

Plant Breeding 




N all the wide range of relations existing between plants 
and mankind, there is not another single fact which 
compares in importance with this, that plants can be 
altered by man to make them fit better his needs or his 
fancy. His accomplishments in this field, indeed, partake of the 
marvelous. Everybody knows the magnificent exhibition type 
of Chrysanthemum, with its superb great globular head of snowy 
incurving petals, well-nigh geometrical in the perfection of its 
symmetry. But does everyone know that it has been created by 
man out of two daisy-like plants smaller and humbler than the 
commonest weed of our hayfields? Likewise, all those strongly 
individualistic types of the same noble flower, the prim little 
pompon, the star-like anemone, the stiffly-correct reflex, the 
shaggy Japanese, and a number of others, a few of which are 
shown clustered together upon the accompanying plate (fig- 
ure 174), have all been differentiated from the same unpromising 
beginning. Again, the Bartlett Pear, huge and luscious, has been 
developed within three hundred years from a small stony fruit 
attractive to no one except vagabonds and omnivorous small 
boys. Indian Corn and Wheat, chief of the food plants of civilized 
man, have been improved so far from the simple wild grasses with 
which the first cultivators had to begin, that Botanists are hardly 
yet fully agreed as to what those wild ancestors were. Oranges, 

426 




FIG. 174. The leading types of Chrysanthemum, all developed by man from wild an- 
cestors having a size and form very like the lowermost single flowers of the picture. 



427 



428 



The Living Plant 



Bananas, Pineapples have been immensely enlarged, greatly im- 
proved in flavor, and actually rendered seedless. Brilliantly red 
foliage plants, including some trees, have been derived from 
green forbears. Invaluable vegetables, often imposing in size, 
have been made to spring from insignificant weeds, as in case of 
the familiar varieties represented in the accompanying picture 




FIG. 175. Representative forms of Cabbage, Kohl-rabi, Cauliflower, Brussels Sprouts, 
and Tree Cabbage, evolved by man from the wild shore plant, Brassica oleracea, of 
which two forms are shown in the upper left hand corner. The pictures are about 
one twenty-fourth of the natural size. (Redrawn from a colored wall-chart by Laurent 
and Errera.) 

(figure 175). Not one of these remarkable productions, or any of 
the vast number of which they are typical, would exist to-day, 
were it not for the craft and the patience of man. It is now our 
particular task to inquire in exactly what way this indubitable 
miracle has been wrought. 

The methods of plant improvement are few, old, simple, and 



Improvements Made in Plants by Man 429 

perfectly known. The earliest cultivators made use of them, and 
the most scientific of horticultural experts have no others to-day. 
For convenience of study we may consider them as three in 
number and distinct, though in fact they are interwoven inex- 
tricably. They are, Selection of Variations; Preservation of 
Sports; Crossing and Hybridization. And perhaps the reader will 
here add in his mind "and also Cultivation;" but he would be 
wrong. Although cultivation can produce better individuals, it 
cannot produce of itself better races, for the two are not the same 
thing at all. 

1. The Selection of Variations. The reader will already have 
noticed the very close connection which exists between Evolution, 
considered in the preceding chapter, and the Improvement of 
Plants by man, or Plant Breeding, our immediate subject, a 
connection which explains the juxtaposition of the two chapters, 
and is not badly expressed by calling Plant Breeding artificial 
evolution. Moreover, the two have precisely the same basis, in 
Variation, which we must now consider rather more fully than was 
needful before. 

If all the plants of one kind, or species, were born exactly like 
one another, as crystals are, then, so far as we can see, no im- 
provement of plants by man would be possible. But plants of 
the same species are not born alike any more than are people of 
the same race or even the same blood. In a field of Corn, are all 
the plants of one height, or have they the same number of grains 
to the ear? Are the Elms in a meadow all cast in the same mold 
of grace? Are the flowers of any one annual precisely alike in 
their hue? There is an experiment which my students try every 
year, with a result that is always surprising to them, and a satis- 
faction to me. From a large lot of seeds of the same variety and 
crop, they select a definite number of grains just as similar as 
possible in size, form, weight, color and other features; these they 
plant at exactly equal distances apart, at the same depths and in 
the same positions, in a box of evenly-mixed earth, which is then 



430 



The Living Plant 



kept watered, warmed and lighted uniformly over its whole sur- 
face. This is something which the reader can readily try for 
himself, and I commend it to his favorable attention. As the 
plants come up, the differences they exhibit in rapidity of ger- 
mination, in the rate of subsequent growth, and in every detail of 
their structure, are most striking, as the accompanying pictures, 
traced from photographs, to some extent illustrate (figure 176). 
In their main features, it is true, those by which we distinguish 




FIG. 17(i. Young seedlings of String Bean and of Corn, grown from seeds as nearly alike 
in all visible features as possible, and planted exactly alike. Traced from a photograph. 

them, plants of the same species are alike, but in their details 
they are always different, and often conspicuously so. Plants of 
the same kind are, as it were, alike in general but different in 
particular. The matter is sometimes expressed in this way, that 
no living being is just like any other living being, a statement 
impossible of logical proof, but shown by experience to be true 
for all practical purposes. 

Turning now to a more exact examination of these differences, 
or variations, we find that they arise from diverse causes. A 
part of them are of purely mechanical origin, being forcibly im- 
posed upon some individuals, but not others, by overcrowding, 
attacks of enemies, or lack of suitable nourishment. Thus, in- 
sufficiency of water causes the dwarfing of plants simply because 
they have not enough water with which to grow big; and man can 
make individual plants short-stemmed in this way if he wants to. 
Another part of the variations arise from the remarkable power 



Improvements Made in Plants by Man 431 

which individuals have of adjusting their parts to peculiarities in 
the distribution of light, moisture, minerals or other essential 
conditions of their immediate surroundings, a power which we 
have studied with some care in the chapter on Irritability. An 
example thereof is the greater lengthening of stems (which are 
" drawn," in the language of gardeners) when exposed to insuffi- 
cient light, and of which the very long shoots formed by potatoes in 
cellars are an extreme example. Upon these stems the deficiency 
of light acts as a stimulus, making them lengthen out as if in the 
effort to carry their leaves into full brightness; and man can make 
plants long-stemmed in this way if he wants to. But when all of 
the differences due to mechanical causes or to irritability are 
eliminated, as can largely be done by careful experiment, there 
remains always a great residue of differences for which there is no 
conceivable origin except that they are innate or inborn in the 
plants themselves. Thus, when all external conditions of water- 
supply, light, &c., are carefully made the same for all the plants 
under our experiment above described, the stems of the plants 
nevertheless differ in length, some being shorter and some longer. 
And man can also obtain long-stemmed plants in this way if he 
wants to. It is thus plain that the differences between individual 
plants of the same species arise in at least three different ways, 
which we may summarize in an order the reverse of that of their 
treatment above, by saying, that in some of their features plants 
are born different, others of their differences are achieved, while 
some of their differences are thrust upon them. 

Of the three kinds of differences, the inborn variations are the 
only ones important in the improvement of plants, as in natural 
evolution, and for the same reason, that they are the only kind 
which can be transmitted to descendants. Although man is able 
by regulation of the water or light supply to make individual 
plants short-stemmed or long-stemmed, he cannot by this means 
make a short-stemmed or long-stemmed race which will reproduce 
itself generation after generation. The only known way in which 



432 The Living Plant 

he can obtain such a race is by watching for plants which naturally 
exhibit an inborn short-stemmed or long-stemmed variation, re- 
spectively, selecting them out and propagating them; the short- or 
long-stemmed character will appear in their descendants, and by 
consistent repetition of selection of the same variation for some 
generations, a race capable of perpetuating the short- or long- 
stemmed character can be obtained. The fact, then, that innate 
variations are hereditary is the most important fact about them 
from the point of view of plant improvement. 

There is one other point about the heredibility of variations 
which we must note at this place. It was Darwin's view that 
variations rise and fall, or flash, as it were, across several genera- 
tions, and, unless preserved by selection, sooner or later die out. 
But modern studies are showing that variations appear linked 
several together in those mutants, biotypes, or elementary species 
which we have already considered in the preceding chapter, 
while, once in existence, they persist indefinitely. What then, 
on the newer view, is the gardener doing when selecting vari- 
ations for the improvement of plants? Simply this, he is 
isolating the desirable biotypes from among the less desirable 
kinds making up the great mixture of which any crop consists. 
Thus a field of Corn or Wheat consists of a great number of bio- 
types, and hybrids thereof, from which the best kinds can be 
selected and propagated. But, on the newer view, once the best 
biotypes are isolated, no further improvement is possible, while 
the selection of variations should permit an indefinite, or at least 
much larger improvement. Experience is certainly showing the 
truth of the modern view in many cases, though the accumulation 
of single small variations seems equally clear in other instances. 

A second great fact about variation is this, it is spontaneous, 
which means that it appears without any determinable reference 
to any features of the surroundings. But while the surroundings 
do not in any known way determine the nature of variations, they 
certainly do promote them, both in number and intensity, as 



Improvements Made in Plants by Man 433 

shown by the fact that variations become more active when the 
external conditions are changed. This happens when plants are 
taken to new countries; or are brought out of forest or field 
into garden or greenhouse; or are subjected to high cultivation, 
through which are provided better conditions for nourishment and 
comparative freedom from natural enemies; or are given different 
soils, fertilizers and exposures; or are crossed in reproduction, 
a matter which we shall consider more fully in a moment. There 
are, of course, limits to the change of conditions that plants can 
endure, but within those limits all changes in external conditions 
arc followed by more rapid, diverse, and extreme variation. Va- 
riation in organisms may be symbolized by the gentle trembling of 
the surface of water held in a full dish at arm's length; if the 
hands are deliberately shaken a little, the trembling increases, 
though the shaking must be kept within limits, else the water is 
spilled from the dish. The cultivators of plants, realizing well 
that variation is the basis of the possibility of improving plants, 
and observing that change in conditions promotes it, have from 
the earliest times made use of these methods for rendering more 
variable those forms which they wish to improve, but which 
naturally exhibit little variation. This is precisely what they 
mean by their expression " break the type." 

A third vital fact about variation is this: it is fortuitous, which 
means that it takes place in any possible part, feature, or direc- 
tion, indifferently, according to chance, and shows no tendency 
to follow any particular lines, excepting in so far as structural 
conditions may make it easier to vary one way than another. 
Stems do not vary towards shortness alone nor longness alone, 
but towards shortness, longness, thickness, thinness, roundness, 
angularity, flexibility, stiffness, and any other peculiarities which 
the construction of stems makes it possible for them to exhibit. 
Moreover, these variations insist, so to speak, upon originating 
and directing themselves, and all the ingenuity of man has not 
yet enabled him either to originate or to determine the direction 



434 The Living Plant 

of any desired variation in any given plant. Therefore his only 
resource is to wait until the desired variation appears, which will 
be the sooner the larger the number of plants that he deals with, 
and the more actively he employs the devices for " breaking the 
type." Under these conditions, it is only a question of time when 
any desired variation that is mechanically, physically, or chem- 
ically possible will appear, after which it can be selected and 
intensified by the methods already described. 

Let us now illustrate, by a suppositional case, the way in which 
man makes use of variation in improving some particular kind of 
plant. Let us suppose that out of a race of white-flowered plants, 
the breeder desires to develop a red-flowered variety. He knows 
it is useless to try to turn the flowers red directly, by chemicals in 
the soil, regulation of the light, or anything of that kind, for al- 
though white flowers might conceivably be made red by such 
methods, the redness would not be transmitted, and the next 
generation would be just as white as ever. He knows that his 
only chance of success lies in the spontaneous appearance of a 
strain of red color, and accordingly he grows just as many plants 
as he can possibly find space for, giving them diverse conditions 
of soil, fertilizers, situation and cultivation, in an effort to break 
the white type. Inevitably, sooner or later, unless, indeed, as 
sometimes happens, there is some chemical obstacle in the con- 
stitution of the plant, some redness will appear, faintly perhaps 
but unmistakably, in some of the white blossoms. He then 
isolates those plants, remorselessly destroying all the remainder, 
and breeds them together if possible, though this is by no means 
indispensable. From the seeds of the selected plants he raises as 
many as he can, and amongst their flowers though some revert 
back to whiteness, the majority are likely to show the red strain of 
the parents, while a few (though perhaps not for another genera- 
tion or two) will exhibit a still redder strain. The latter, of course, 
are then selected, and bred together, and their seeds are sown as 
before. In the resulting generation will appear fewer white 



Improvements Made in Plants by Man 435 

flowers than before, a larger proportion of red like the parents, 
and perhaps a still redder strain, though again this may not 
appear for a number of generations. Thus, gradually, generation 
after generation, the quality of redness becomes extended and 
intensified, while whiteness diminishes to final disappearance; so 
that finally a permanently red-flowering race has been secured. 
Whether, however, this process depends chiefly upon the selection 
and accumulation of red variations in the Darwinian sense, or 
upon the isolation of successively-appearing new biotypes, I do 
not know, but expect the near future to decide. 

In any case, the improvement of plants by the selection of 
variations is a slow process, and it is fortunate that a far more 
rapid, even though rather spasmodic method exists, viz., the 
preservation of sports. 

2. The Preservation of Sports. The most of my readers, I fancy, 
know something about sports among plants. The most typical 
ones originate like this. On some ordinary plant a single bud, 
differing visibly in no wise from its neighbors, grows out to a 
branch which bears leaves, fruits or flowers conspicuously dif- 
ferent from all others on that plant. About five years ago, in 
a greenhouse where I teach, a certain Pompon Chrysanthemum 
bearing pretty pink flowers put forth a single branch on which 
all of the flowers were entirely different, being a striking bronze 
brown. For that season the plant was a wonder to visitors, who 
delighted to represent that they could hardly believe their eye- 
sight; a pride to the students, who accepted its appearance as a 
delicate compliment to themselves; and a treasure to me, who 
made the best of this unusual educational opportunity. From 
both the sporting and the ordinary branches we took cuttings, 
and from these the next season we grew two plants of the re- 
spective colors, which we have propagated continuously to this 
day. On an ordinary green beech tree in Scotland somewhat 
less than a century ago, a single one of the innumerable buds 
grew into a branch on which every leaf was dark red. That 



436 The Living Plant 

branch was propagated by grafting and these plants by graft- 
ing again; and such was the origin of all those favorite lawn 
trees which we call copper beeches. On an ordinary orange 
tree, not so many years ago, a single bud produced a branch 
which bore only seedless fruit, the seedlessness being correlated 
with the presence of a tiny accessory orange embedded almost 
wholly in the larger; that branch was grafted, as were the result- 
ing branches; and this is the origin of the thousands of trees now 
bearing the navel orange. Nectarines are bud sports from peach 
trees, and most of our finest varieties of apples, of pears, and of 
many other fruits have originated in just such a manner. Haw- 
thorns, Azaleas, Pelargoniums ("Geraniums"), Roses, Carna- 
tions, and many other plants sport greatly in the color of their 
flowers; and many of our choicest varieties of these charming 
plants, including double-flowered forms, came thus into existence, 
as did many of our cut-leaved trees, variegated plants, and 
crested oddities. Sometimes the sports take curious directions, 
as in the case of branches which bear leaves that unfold in the 
spring several days in advance of any others on the tree. I know 
a tulip bulb which, year after year, produces flowers highly 
doubled and accompanied by a colored peduncular bract. This 
latter case is interesting as marking a transition over to those 
peculiar structures called monstrosities, which are largely, though 
not always, sports. Indeed a monstrosity is usually but a sport 
which strikes us as somewhat abnormal or "queer," such for 
example as green roses, crested plants, and other "freaks," all of 
which can be propagated regularly by cuttings. And other 
sports are known in great number as recorded in the books devoted 
to horticulture. But as to this, one must not forget that only 
those sports which appeal to man as useful, attractive, or curious, 
are likely to receive mention in such books, while innumerable 
others, which make their appearance but have no interest to man, 
are left in oblivion. 
Concerning the causes of bud sports we know as little as we do 



Improvements Made in Plants by Man 437 

concerning the causes of variation, the two, indeed, being doubt- 
less identical in nature. Like variations, bud sports are spon- 
taneous and fortuitous and can be rendered more frequent by 
cultivation; and they are hereditary through the new buds they 
produce, though never by seeds. Everybody knows that a 
Bartlett Pear or a Baldwin Apple can be propagated by grafting, 
but not by the seeds, which do not produce those fruits, but just 
plain ordinary mongrel pears and apples. Were it not for the 
fact that most of the plants which produce bud sports can be 
propagated by slips, or cuttings, or else by grafting (which is 
merely a process of giving ready formed roots to slips unable to 
make roots for themselves), it would not be possible to preserve 
bud sports, and they would perish with the plants which produce 
them. But those methods of propagation do permit man to 
preserve them, to his very great advantage. 

Sports from buds, however, are not the only kind, for seed 
sports also occur, though upon the whole they are less conspicuous 
and important than bud sports. Among brilliantly red cardinal 
flowers, some plants occasionally occur with pure white flowers; 
when seeds from the white flowers are planted, they produce 
white-flowering plants. The white cardinal flowers are typical 
seed sports, and the fact that they propagate by seed is typ- 
ical. Some cut-leaved trees (e. g. Wier's cut-leaf Maple), and 
some fine varieties of fruits have also originated as seed sports. 
Whether these trees come true to seed I do not know, and the 
matter is not practically important, since they can be propagated 
far more speedily and easily by grafting. It is obvious that seed 
sports which propagate their characters through seeds differ in 
no essential particular, except perhaps that of degree, from the 
mutants, or biotypes, described in the chapter on Evolution. 

3. Crossing and Hybridization. The reader will recall that seed- 
formation must be preceded by fertilization, which in turn re- 
quires that the pollen-grain containing a male cell shall be trans- 
ferred from anthers where it is made, to a stigma giving access 



438 The Living Plant 

to the female cell in the ovule. Now in nature this transfer is 
effected by the agency of wind, water, or insects, but in culti- 
vation man can transfer the pollen himself if he pleases, and 
can thus to some extent control the parentage of the new in- 
dividuals formed in the ovules. And these are the various com- 
binations he can make : 

(1) He can pollinate a given stigma by pollen from the same 
flower. This is called close pollination in Botany and in-breeding 
in Horticulture. With many plants, perhaps the majority, no 
result follows, for the seed does not set, the ovule being sterile to 
the pollen of the same flower; while in many kinds of flowers such 
pollination is impossible, because the pollen and ovules in each 
single flower ripen at different times, or because of other im- 
pediments. It is thus obvious that nature takes trouble, so to 
speak, to prevent such in-breeding; and the implication that such 
breeding is in general not advantageous is confirmed by such 
evidence from experiment as we possess, which shows that the 
offspring of close in-breeding are generally inferior in variability 
if not in vigor to those more widely bred. But in the very fact 
that in-breeding does not favor variability is found its chief horti- 
cultural importance, for it can be used to keep a race true to its 
type when that is desirable. In practice, however, such use is 
very limited because the same result can be attained much more 
easily in most plants by propagation through cuttings or grafting, 
and by the systematic weeding out of all plants (horticulturally 
called "rogues") which show individual variations. 

(2) He can pollinate a given stigma by pollen from another 
flower on the same plant. This also is in-breeding, and experi- 
ments have shown that the results are little if any better than in 
the case of the first method. 

(3) He can pollinate a given stigma by pollen from a different 
plant of the same kind. This is called cross pollination in Botany, 
and crossing in Horticulture, and is that to which most of the 
cross-pollinating mechanisms and methods in flowers are adapted. 



Improvements Made in Plants by Man 439 

It is both known from experience, and also has been shown by 
experiment, that crossing yields more vigorous, abundant, and 
variable offspring than in-breeding, in which fact lies the reason, 
doubtless, why nature promotes it. In crossing, not only is va- 
riability promoted by the introduction of the peculiarities of two 
lines of ancestry, but the very commingling of the two strains of 
protoplasm seems to favor the appearance of wholly new varia- 
tions, somewhat after the analogy of these liquids in chemistry, 
which are perfectly clear by themselves but turbid when com- 
mingled. It appears also to be a fact that the offspring are more 
vigorous, prolific, and variable yet, if the two plants between 
which the cross is made are not raised side by side under the 
same conditions, but apart and under somewhat different condi- 
tions. Cross pollination between plants thus grown is something 
which nature cannot provide for, but man can, and sometimes 
does, as when he plants seeds in alternate rows treated somewhat 
differently in cultivation. By this method man can intensify 
variation, and thus provide a wider and better basis for selection. 
(4) He can pollinate a given stigma by pollen from a plant of 
another variety of the same species. This is called hybridization 
in both Botany and Horticulture, though by some it is also desig- 
nated crossing. Occasionally no result follows, but usually seed 
sets and will grow into new plants which breed freely together. 
The first generation of such hybrid progeny exhibit characters de- 
rived from both of the original parents, and are likely to be more 
vigorous than those parents; but (and this will be news to many 
of my readers), these characters are not combined in the same 
way in all of the descendants of those hybrids, though the ways 
are very definite. This very important matter, which involves a 
notable natural law discovered by Mendel and known by his name, 
has already been considered in the chapter on Reproduction, and 
it will suffice to recall here that the characters of the original 
parents are inherited by the descendants of hybrids as definite 
entities in definite mathematical proportions. This fundamental 



440 The Living Plant 

fact has a practical consequence of the first importance, since it 
is thus made possible for a breeder so to breed the hybrids to- 
gether as, on the one hand, to eliminate utterly out of the hybrid 
race any given undesirable quality inherited from either of the 
original parents, and, on the other, to combine and fix perma- 
nently in the race any two given desirable qualities originally 
occurring in separate parent races. Thus, it has been possible 
to produce hybrid races of wheat in which the superior flour- 
producing quality of one variety has been united with the superior 
frost-resisting quality of another, the inferior frost-resisting 
quality of the former, and the inferior flour-producing qualities 
of the latter having been eliminated permanently out of the 
hybrid race. Moreover, it is possible, theoretically at least, thus 
to combine in one race any number of good qualities from any 
number of different varieties of a species, though in practice, as 
we shall see in a moment, the matter is attended with immense 
practical difficulties. It is, however, in this possibility of com- 
bining in one race the desirable qualities from different races 
while eliminating the opposite qualities, that the highest utility 
of hybridization in connection with the improvement of plants 
consists. 

(5) He can pollinate a given stigma by pollen from a plant of 
another, but allied, species. This also is called hybridization, in 
both Botany and Horticulture. In the vast majority of such 
pollinations no result follows, but in the few cases where seed is 
formed, the derived specific hybrids, like the varietal hybrids 
just considered, exhibit characters derived from both parents, as 
also new characters not traceable to either. Like the first genera- 
tion of varietal hybrids, also, they are often larger and more 
vigorous than either parent; but on the other hand they are al- 
most invariably defective in reproductive power, and can hardly 
ever reproduce by seeds. A famous example of a species hybrid is 
Lilium Parkmanni, a magnificent Lily even finer than either of its 
superb parents, but it is rarely seen in gardens because it does not 



Improvements Made in Plants by Man 441 

reproduce at all by seeds and only badly by bulbs. Such of these 
species hybrids, however, as can be reproduced by cuttings or 
grafting can be preserved indefinitely, and this is the case with 
hybrid trees, because the seedling can be grafted into either of the 
parent trees, or into some allied kind, and thereafter can be multi- 
plied with rapidity and certainty. It is obvious from these con- 
siderations that specific hybrids can only rarely be made the 
foundation of a race, and equally plain that they can play no 
appreciable part in the natural evolution of plants. 

(6) He can pollinate a given stigma by pollen from a plant of 
another but allied genus. Though some such generic hybrids 
have been made as a matter of scientific experiment, this has 
only been possible with genera extremely closely related, and 
moreover the hybrids are unstable and of no horticultural im- 
portance. Nor have any attempts at hybridization over wider 
limits ever succeeded; and the occasional newspaper accounts of 
crosses between members of different plant families are lies, when 
they are not obvious jokes. 

It is evident from this discussion that plant-breeders make use 
of in-breeding, crossing, and hybridization for various purposes in 
accordance with the results which they wish to attain. By suit- 
able combinations it is possible to keep races close to their type 
and thus preserve desirable characteristics; to break the type and 
thus provide a basis for the development of new characteristics 
through selection; to eliminate undesirable features out of a race; 
to combine the desirable features of two or more races into one; 
and in general to promote vigor and productivity. I think it will 
now be evident why crossing and hybridization are so prominent 
in plant improvement. 

It will interest the reader, at this point, to learn in what way 
crossing and hybridization are effected in practice. It is no 
trouble at all to transfer the pollen from any ripe stamens to any 
ripe stigma. It is only necessary to pick the fine pollen dust from 
the opened anthers by some dry utensil to which it will cling (for 



442 The Living Plant 

which purpose a common camelshair paint brush is admirable 
and usually employed), and then press it against the desired 
stigma, which is sticky arid to which therefore the pollen adheres. 
The difficulty in the process comes from the fact that undesired 
pollen may also reach the stigma, and effect a wrong fertilization. 
It is therefore necessary to prevent access of any pollen except 
that deliberately placed upon the stigma by the experimenter. 
Close pollination is prevented, in those plants which allow it, 
by snipping off the stamens before the anthers are ripe; while 
undesired cross pollination is prevented by use of a gauze, or thin 
paper bag kept closely tied over the flower except at the moment 
when the desired pollen is applied to the ripe stigma. The sight of 
such bagged plants must be familiar to all those who have visited 
agricultural experiment stations, and they are shown in figure 173. 

The reader will no doubt be surprised that in this discussion I 
have laid no more stress upon cultivation, which surely, he will 
say, does much improve plants. Cultivation consists in giving 
to plants such conditions of space, nourishment, and freedom 
from enemies as will permit them to develop to the highest degree 
that their internal capacities allow. It produces, therefore, better 
individuals and crops. But it does not produce better races, 
because, as we know, the good effects of cultivation are chiefly 
irritable responses whose results are never transmitted to the next 
generation. Indirectly, however, cultivation does help in racial 
improvement, for on the one hand all offspring are benefited by 
greater physical health in their parents, and, on the other, with 
greater physical vigor goes greater variability and tendency to 
production of sports, those foundations of the improvement of 
races. Just as the best nourished animals play more vigorously 
than the ill-nourished, so the best cultivated plants vary and sport 
the most actively, from very excess of physical vigor, no doubt. 

In my discussion of this subject thus far, I have made it an 
aim, as elsewhere through this book, to exhibit the theory, so to 
speak, of the subject. For this purpose I have had to separate 



Improvements Made in Plants by Man 443 

out the constituent methods and discuss each by itself. But 
thereby I have given the matter an aspect of simplicity which 
it is far from deserving, for not only are the methods inextricably 
interconnected, but practical difficulties of innumerable sorts 
interpose large obstacles to their successful operation. Thus, 
I have spoken of plant breeding as it would be conducted when a 
matter of deliberate intention on the part of a worker with a 
definite idea in his mind. This, however, it rarely is except in the 
case of modern scientific plant-breeders, wealthy amateurs, or a 
few far-sighted commercial dealers in horticultural novelties, of 
whom the most conspicuous by far is Luther Burbank, well known 
of late to the readers of periodical literature. As a matter of fact, 
most plant improvement has been made on the spur of the mo- 
ment, by the selection of something which happened to please 
the fancy, or appeal to the sense of profit, of gardener or farmer, 
who of course has always sought to propagate from the plants he 
considers his "best. " But the art of horticulture is long, and the 
life of man is short, and fancies change, and things that are profit- 
able vary; wherefore improvement has been spasmodic, and 
along most devious courses. Nor are horticultural productions 
wholly stable when once secured, for varieties, even when true to 
their good character for a time, tend strongly to revert or merge 
off or "wear out" to less desirable kinds, though there is perhaps 
a difference between mutations which are permanently stable, and 
the results of the selection of small variations, which are unstable. 
Furthermore, hybridization, especially for the combining of 
features from different races into one, is by no means so simple as 
its theory implies, but a process distinguished by innumerable 
failures, and requiring a persistence and skill that few breeders 
command. The potentialities of improvement, indeed, have a 
vast burden of practical troubles to carry; and it is this which 
makes its progress so halting and laborious. 

It is evident that in his improvement of plants, man never 
creates, except by a figure of speech, but only directs. He cannot 



444 The Living Plant 

compel plants to go as he wishes, but he can lead them in any 
direction they are capable of going. The forces of improvement 
lie deep inside of the plants themselves, seething and smoldering 
ready for an outbreak; all that man can do is to suppress or bank 
them in places where they are doing no good, and give them free 
vent where they can produce beneficent results. The method of 
effecting desired results through the guidance of internal forces is 
not, however, confined to plant breeding, but is that by which all 
great organizers of large enterprises, and all great leaders of men, 
effect their successes. By this method they succeed when those 
who try to force the improvement from without meet only with 
failure. 




CHAPTER XVIII 

THE PRINCIPAL GROUPS INTO WHICH PLANTS NAT- 
URALLY FALL, WHETHER BY RELATIONSHIP OR 
HABIT 

Classification 

|]NE does not go far with the study of plants before he 
perceives that they fall into groups, and groups within 
groups, according to the degrees of their likenesses and 
differences. Some kinds are so closely alike that 
botanical experts dispute as to whether they really are different 
or merely two forms of the same, while others are so very unlike 
that they offer not the least point of resemblance; and there is 
every gradation between. The arrangements of plants in their 
groups, and of these in relation to one another, is Classification, 
which we must now proceed to consider in so far as it has connec- 
tion with the particular theme of this book. And we naturally 
begin with the groups which are largest and best defined, of which 
there are five, Algae, Fungi, Moss-Plants, Fern-Plants, and 
Seed-Plants, 

The Algae. These are the distinctive plants of the waters, 
comprising especially the Seaweeds, but also many kinds that 
dwell in rivers and lakes, and a few that live out in the air. In 
size they range widely, from kinds too small for the eye to detect 
up to the great Macrocystis of the Pacific, whose thousand feet 
(pretty nearly) of length surpasses anything that land plants can 
offer. In shape they are bewilderingly multifarious, spheres, 
cylinders, hairs, plates, tufts, fronds, and even leafy stems, which 

445 



446 The Living Plant 

latter bear a striking, albeit superficial, resemblance to those 
parts in the higher land plants. Whatever their shapes, they 
exhibit a wide prevalence of minuteness, thinness, or fine division 
of structure, these features being correlated with the comparative 
scarcity of the indispensable gases, which they, like the fishes, 
must take from solution in the water. In color they are typically 
green from the presence of chlorophyll, by aid of which they make 
their own food precisely in the manner of the familiar green land 
plants; but in a good many kinds, including most of the larger 
and best-known, the green is mixed up with red or brown pig- 
ments which aid in a better utilization of the light under the con- 
ditions prevailing where those kinds make their homes. Their 
anatomical structure is cellular, as in land plants, but much 
simpler, with far less division of labor among the various cells, 
and only unimportant structural differences between the several 
tissues. Their reproduction is partly by fission, but chiefly by 
spores, which are simple one-celled bodies various in aspect and 
mode of formation, some of them actively free-swimming, and 
others passively floated by currents of water; in addition, fertiliza- 
tion occurs, in all grades of complexity from the accidental fusion 
of two precisely-similar free-swimming cells up to the union of a 
tiny, free-swimming, chemotropically-attracted, male cell with a 
sessile food-filled female cell. 

The best-known kinds of the Algae are these. Among the 
GREEN (AND BLUE-GREEN) ALGAE are the Diatoms, found in all 
waters, with microscopical flinty shells of wonderful beauty and 
marvelous variety; the Blue-green kinds, forming unhealthy- 
looking scums of that color in unpleasant damp places; Pleuro- 
coccusj which makes up the familiar green coating upon the shaded 
sides of standing tree-trunks; Vaueheria, the darker-green coating 
on damp earth in warm shaded places; Uroglcena, hardly visible to 
sight, which gives the bad odors and taste to the water of reser- 
voirs, from which, fortunately, it can be driven by traces of com- 
pounds of copper; Spirogyra, which composes the very bright green 



Groups into Which Plants Naturally Fall 447 

felted mats, buoyed up by entangled bubbles of gas on the surface 
of still waters; Cladophora, and its relatives, often mistaken for 
Mosses, those hair-like, net-like, brush-like, fringe-like forms 
which sway and wave from their moorings on stones in the bot- 
toms of slow-moving brooks; and certain of the simpler kinds 
which are enslaved in the meshes of some Fungi to make up the 
remarkable Lichens. The curious Red Snow, reported by Arctic 
and Alpine expeditions, and the redness of the Red Sea, famed in 
geography and biblical history, owe their characteristic colors to 
certain red stages in the development of simple Green, or Blue- 
green, Algae. 

Of the BROWN ALGAE the most familiar are the Rockweeds, 
whose tough branching fronds cover rocks of the beaches where 
exposed to the swing of the tides; the great leathery Kelps, 
known to the sailors as " Devil's Aprons," abounding in the 
seas of the north; and the leafy-stemmed kinds, including the 
Sargassum which gives name to a Sea, more plenty towards the 
south. These Brown Algae are the only marine kinds which are 
exposed with regularity to the air, either between tides on the 
beaches or during flotation on the surface; and this better access 
to gas-supply helps to explain their larger and stouter forms. 

Of the RED ALGAE, the best known are the dark-red, almost 
purple Irish Moss and Dulse, familiar to all persons who have had 
the good fortune to grow up in a sea-port; the Corallines, those 
reddish-chalky-warty incrustations upon stones near low-tide 
mark, often mistaken for corals which they aid materially in the 
building of coral reefs, though also extending far north of the 
range of those much misunderstood animals; and the beautiful 
rose-red, soft-foliaged Sea-mosses, most plenty towards the south, 
where they often arouse the collecting instinct in persons who 
never have been moved to collect anything else. 

Such are some of the principal ones of the fourteen thousand or 
more different kinds of Algae which botanists have named and 
described. 



448 The Living Plant 

As one might expect, there are plants supposed to be Algae 
which really are not. Thus the Eel-grasses, the Pond-weeds, the 
Duck-weeds, and many other Water-weeds, are Flowering Plants 
which have adopted a life in the water, and therefore an Alga-like 
aspect. They can be told by their flowers which they bear in 
their season, and which separate them sharply from the spore- 
bearing Algae. 

We turn now to consider the origin and evolution of these 
Algae, together with their classification. If evolution is a fact, 
and all evidence appears to agree that it is, then classification 
must be an expression of genealogical descent, and expressible in 
a genealogical tree, comparable with the kind which some people 
are fond of constructing to show the genealogical ramifications of 
human families. Such a tree, for the great primary groups and 
their principal subdivisions, is presented in our accompanying 
diagram (figure 177), and the mode of its construction is as follows. 

First as to its most ancient, or lowermost, part. We have good 
reason for believing, as the chapter on Protoplasm suggested, that 
our present green plants were preceded in time by a colorless 
kind, which, though without chlorophyll and of the utmost 
simplicity, could yet make their own food from carbon dioxide 
and water by using the energy of chemical oxidation of soil min- 
erals in place of that of the sunlight. We have precisely such 
chemosynthetic organisms, a kind of soil Bacteria, still living on 
the earth at this day; and they are doubtless the lineal descend- 
ants of the ancient forms, which probably lived in the mud of 
shallow seas that may be full of them yet. These ancient chem- 
osynthetic organisms were neither animal nor plant but both and 
between, the dawn of the kind of plant-animal forms sometimes 
called Protista; and therefore I suggest that we call them Eo- 
protista. These Eoprotista, therefore, form the base of the gen- 
ealogical tree. Then, like all later groups, they must have ex- 
panded, developed, varied, evolved, thus originating a great 
many branches, of which the greater number perished, and only 



Groups into Which Plants Naturally Fall 449 



four survived; (a) the Chemosynthetic Bacteria, whose persistence 
to this day is shown by the continuous line sweeping up and off to 
the outer rim of the tree where lies the vegetation of this, our own, 



Gamopctalous 
Dicotyledons 



Polypetalous 
Dicotyledons 

Apctalous Trees 

Conifers 
Cycads 

Club- 
Mosses 
Ferns 

Horse- 
tails 




Mosses 



Slime 
molds 



Monocotyledons 



ttasidia Fungi 



Sac Fungi 

Yeasts 
Red Algae 



Brown Algae 
Green 

Algae 
Algoid 
Fungi 
Blue-green 

Algae 
Bacteria 
Chemo- 
synthetic 
Bacteria 



Eoprotista. 

FIG. 177. A genealogical tree of the principal groups of plants. The axial lines show the 
supposed relations of the groups at the time of their original evolution from one an- 
other, while the solid trunks show their present numbers and connections. 

day; (b) the Animals, a vast group, shown on the left by an un- 
finished stump which it is some zoologist's business to finish if 
he wants it; (c) the Slime-molds, well described by their name, a 
group of very simple organisms which creep as white films over 



4So The Living Plant 

damp rotting wood in dark places, in a way so like to some animals 
that zoologists lay even stronger claim than do botanists to their 
possession, and, (d) the most important of all, the Algae. 

So, the Algae evolved probably from Eoprotista, and by a 
method which was somewhat like this. Among the variations or 
mutations (or whatsoever else it is that our chapter on Evolution 
concluded does originate innovations in living Nature) arising 
in the Eoprotista, must have been many new chemical com- 
pounds, among which, in time, appeared chlorophyll. This sub- 
stance happening to possess such properties that sunlight falling 
upon it dissociates carbon dioxide, enabled its possessors to make 
their food far more rapidly and easily than by the old chemosyn- 
thetic method; and therefore those plants were enabled to grow, 
increase, develop, and expand immensely until they filled the 
lighted seas of the world. Thus the little chlorophyll-bearing 
branch of our tree, the one that happened to be thus fruitful 
among so many that were barren, expanded so greatly that 
gradually it became the main trunk of the tree, which fact we 
may express by swinging it around into the main line of ascent as 
has been done in our diagram. Thus arose the Algae, the char- 
acteristic group of the waters, in which they have persisted right 
down to the present, giving origin in time to Red and Brown 
branches as the tree represents. It is interesting to know that 
our living Algae have an ancestry so ancient, so ancient indeed 
that they have doubtless had time to evolve everything of which 
they are capable, and have consequently reached a condition of 
comparative evolutionary stability. 

The Fungi. These are, so to speak, the degraded and criminal 
classes of plants, which prey upon good plant society, or eke out 
an unenviable existence as scavengers of its offal. Expressed 
more precisely, in the manner of science, they are parasites which 
take all their food ready-made from living green plants or from 
animals, causing, incidentally, damage, disease, or death; or else 
they are saprophytes whch consume and destroy dead animal or 



Groups into Which Plants Naturally Fall 451 

^ 

plant remains, thus turning them back into the general circula- 
tion of nature and rendering a service to the remainder of living 
things. This dependence upon other organisms for their food, 
with the correlated absence of chlorophyll, is their one great dis- 
tinctive feature. 

The principal kinds of Fungi are these: Bacteria, commonly 
called "germs," or " microbes, " tiniest of living things, some of 
them harmless, others useful, and others the causes of deadly 
diseases; Yeasts, but the reader knows what they do; Molds, 
which spring up on moist bread, preserved fruits, and other good 
materials, spoiling them quickly for use; Mycorhiza, which form 
caps of closely-felted threads over the ends of some roots, and aid 
them to absorb materials from the soil; Water-molds, which form 
the white haloes round dead insects or small fish in the water; 
Blights and Mildews, showing as powdery or woolly white fuzzes 
on grape leaves and others; Rots, which soften, discolor, and ruin 
potatoes and other vegetables and fruits; Spots, which darken 
round areas on various leaves; Smuts, which convert ears of 
grain to an unctuous black powder; Rusts, the ragged red spots 
which appear on the leaves of the wheat in over-wet seasons, and 
on other grains also, to their infinite damage, but which are dear 
to the botanical teacher because of their heterogeneously poly- 
morphic ontogeny; Mushrooms, which are good to eat, and Toad- 
stools, which are not; Puff -balls, whose names sufficiently describe 
them; Black-knots, which form swellings on branches of Cherries, 
with many destructive diseases of Chestnuts and other large 
trees; Bracket-fungi, which appear on the outside of tree-trunks 
as a kind of crude hemispherical bracket, unfortunately, however, 
with the flattened side down; the Lichens, gray, crisp, brittle, and 
crusted, living on rocks, fences and tree-trunks, and deriving their 
food from certain kinds of small Algae which they hold enslaved 
in their meshes; and a great many others not familiar to the 
public but well known to botanical students. 

In shapes the Fungi are even more diversified than the Algae, 



452 The Living Plant 

but they show, for the most part, a double structure imposed by 
their habit of life. First, they possess a feeding body, called a 
mycelium, consisting as a rule of innumerable fine, slender, white 
threads ramifying and radiating everywhere throughout the 
accessible tissues of the living plants, or amongst the decaying 
materials upon which they live; and second, they possess a com- 
pact spore-forming body which comes to the surface, and thus 
carries the spores to a position where they can be scattered by the 
wind. Most of the Fungi familiar to us, such as Rusts, Bracket- 
fungi, or Mushrooms, are simply the spore-forming bodies of 
feeding mycelia which branch profusely, though invisibly, through 
green tissues, tree-trunks, or earth. And it is an interesting 
speculation, by the way, whether kinds like the Bacteria, whose 
structure and habit do not permit them to bring their spores 
thus to the surface for dissemination, may not cause the death 
of their hosts as an adaptive measure in order that their spores 
may be set free by the decomposition of their victims. The 
cellular anatomy of the Fungi differs in a curious particular 
from that of the Algae and other kinds of plants, for the habit of 
forming the thread-like feeding mycelium persists in the spore- 
forming body, which is simply a collection of compacted and 
parallel cellular threads; and this explains why it is that Mush- 
rooms, for instance, break apart in the fibrous-grained way that 
they do. In size the Fungi are all rather small, ranging from 
minute-microscopic up to the Toadstools and the Bracket-fungi, 
which never exceed some two feet across, though to the size of the 
spore-forming bodies must be added that of the radiating myce- 
lium, which may range, albeit tenuously, over a diameter of several 
feet. In color the Fungi are not green, at least of the chlorophyll 
shade, for their most distinctive feature is the total lack of that 
substance; but they are typically white, verging to gray shades or 
brown. The spore-forming bodies, however, are brilliantly 
colored in yellows, purples, and reds in some kinds, notably the 
Rusts and the Smuts, and especially some of the poisonous Toad- 



Groups into Which Plants Naturally Fall 453 

stools, though it is not yet certainly known what the significance 
of these colors may be. The reproduction of the Fungi is multi- 
farious, but most commonly by tiny spores; and these are spread 
by the wind with the dust, of which they make up no inconsider- 
able part. These dust-like spores can be seen en masse by the 
reader if he will place a fresh mushroom, gills down, on some paper; 
for after a few hours a striking spore-print of the gills will appear. 
Spores, furthermore, are often of a thick-walled " resting" sort, 
which can endure dryness, heat, light, and other unfavorable 
conditions for months or even years; and this fact helps to explain 
why those particular plants are so ubiquitous and irrepressible. 
But they also reproduce sexually, at least some of them do, and 
usually by methods so closely like those of the Algae as to suggest 
a relationship between these two groups. This, indeed, is a con- 
clusion sustained by abundance of evidence; and it all goes to 
show that the Fungi are really nothing other than Algae which 
have taken to a parasitic habit of life. 

With the Fungi are commonly reckoned some plants which are 
fungus-like, but not Fungi. Thus the Dodder, and the Indian 
Pipe are Flowering Plants, though they have no chlorophyll or 
leaves, and present a markedly fungus-like aspect in correlation 
with the parasitic or saprophytic habit they have assumed. And 
there are many other flowering parasites in all degrees of develop- 
ment of the habit, as witness the Mistletoe, which is only a part- 
parasite, a kind of a natural graft which takes water and minerals 
from its host, but makes its own food by means of its own chloro- 
phyll. Such plants can always be told by their flowers, which 
they bear at some time in their lives, and which, of course, are 
wholly absent from the Fungi. 

However ignoble the habit of the Fungi may appear from the 
view-point of green plants at whose expense they exist, their 
manner of life has been a success; for it has enabled them to 
develop no less than some sixty-six thousand different kinds al- 
ready known and described by Botanists (between four and five 



454 The Living Plant 

times as many as of Algae), while there doubtless remain a great 
number still to be discovered. 

We turn now to consider the place of the Fungi in our tree of 
descent (figure 177). It seems perfectly clear that they all are 
derived, either immediately or remotely, from the Algae. We can 
imagine that as the Algae became large and abundant, some kinds 
took to growing upon others, at first merely as a convenient situa- 
tion, but later making use of the decaying remains. But in 
nature, as in human affairs, it is only the first step which counts, 
and the transitions from a dead to a dying, then to a sickly, and fi- 
nally to a healthy host are easy, giving origin in turn to an epi- 
phytic, saprophytic and, finally, parasitic mode of life. Then, as 
the Green Algae evolved into the higher and air-livirig forms and 
came out to live on the land, they were accompanied by these par- 
asitic Algae, which gradually became more and more altered in 
adaptation to the new conditions of their existence. And there 
you have the Fungi, which are nothing but parasitic Algae, al- 
though in some cases with an ancestry so ancient that we can 
hardly trace a sign of their primitive origin. The various principal 
sub-groups of the Fungi, the Basidia division to which the Mush- 
rooms belong, most ancient and specialized of them all, the Sac 
Fungi, which include the Lichens, the Algoid Fungi, which com- 
prise the water forms and others that are most like the Algae, are 
shown in our tree in conjunction with their most probable an- 
'cestral branches of the Algae. 

The Moss-plants, or Bryophytes. These are typically the carpet 
plants of the land, especially the woods, where they form the 
fine close covering over ground, boulders, and prostrate tree- 
trunks; but they also extend out beyond into places that are open, 
particularly where wet. They comprise two well-marked divi- 
sions. First are the Liverworts, which mostly lie flat on the 
ground outspread in small thin fronds suggestive strongly of some 
kinds of Algae, though others bear delicate leaves. Second are 
the true Mosses, much more familiar, which have upright, slender, 



Groups into Which Plants Naturally Fall 455 

fine-leafy, low stems growing densely-compacted together, with 
slender-stalked spore cases standing out from their tops. Most 
striking of them all are the Peat-mosses (Sphagnum), which form 
in wet northern climates the great bogs such as, doubtless, 
long ago, played a part in the origination of the coal beds. In 
size, the Moss-plants are all low, not over a few inches in height, 
and they have no parts underground excepting some water- 
absorbing hairs, which fact explains why all Mosses are so 
easily stripped from the ground. Their cellular anatomy in the 
best developed forms includes a waterproof epidermis with 
stomata, and intercellular spaces, features correlated with their 
air-living habit; but otherwise the tissues are little more special- 1 
ized than in Algae, and their lack of a particular strengthening 
and conducting system explains why they never can rise much 
above the ground. In color they are typically green, often intense 
in its shade, from the presence of chlorophyll with which they all 
make their own food, though the greenness is often obscured, 
especially in those of exposed places, by screens of red or brown 
pigments which are doubtless a protection to the protoplasm 
against the injurious action of untempered light. Their reproduc- 
tion is partly by dust-like spores, scattered from exposed spore 
cases by the wind, and partly by fertilization, effected by the 
fusion of a free-swimming male cell with a well-enclosed and pro- 
tected egg-cell. And it is a fact of great interest to Botanists 
that fertilization and the production of spores alternate regularly 
with one another in two separate generations, whereby hangs a 
remarkable tale, too special for relation in this place, and of 
which, moreover, the exact point is still tantalizingly elusive. 
As to the numbers of the Moss-plants, some seventeen thousand 
kinds have been described, and doubtless a good many are still to 
be found. 

Of course there are plants which look like this group, but are 
not. Thus there is a " Liverwort " which is a Flowering Plant 
(the Hepaticd), while the " Spanish Moss/ 7 of the Oaks iix the 



456 The Living Plant 

south, is also a Flowering Plant (belonging to the Pineapple 
Family); but the somewhat similar tree moss, or "Long Moss" 
of the northern woods ("The murmuring pines and the hemlocks, 
bearded with moss . . . stand like Druids of eld") is a Lichen, 
as is the "Reindeer Moss" of the far northern plains. The "Sea- 
mosses" are Algae, as we have seen, and so are a lot of the moss- 
like little plants of fresh waters. Then the creeping Ground Pine 
of our woods, known even botanically as a "Club-moss," is not 
a Moss at all but a Fern-plant, of the group next to be studied. 
Furthermore, even Flowering Plants, especially in open moun- 
tainous regions, but including some kinds nearer home, like the 
Pyxie, assume often the moss habit, and therefore the moss aspect, 
to a degree often completely deceptive were it not for their tell- 
tale flowers which appear at some season. 

We turn now to the place of the Moss-plants in our tree of 
descent (figure 177). There is no question as to their origin from 
the Algae, which, among a great number of branches, must have 
given rise to one with a structure permitting the absorption of 
gases from the air instead of the water. Thus was opened up to 
those plants an immense new field not then possessed by any 
other plants whatsoever, all the surface of still waters and the 
moister parts of the land, which latter were then, it is likely, far 
more extensive than now. Over the land, accordingly, these 
plants proceeded to expand as a dense living carpet, then the 
most conspicuous part of the earth's vegetation. So, our diagram 
shows their particular branch swinging into the main trunk, 
thereby displacing the Algae to a lateral limb; and from that time 
to the present these Moss-plants have persisted supreme in their 
own situation, giving off, however, from the simpler Liverworts 
the more complicated Mosses. 

The Fern-plants, or Pteridophytes. These are typical under- 
growth plants, most at home in the shade of the woods, where they 
occupy a place above the carpet of Moss-plants, and beneath the 
canopy of the forest, though like all other groups they reach far 



Groups into Which Plants Naturally Fall 457 

out beyond their own particular situation. They exhibit three 
main divisions. First are the true Ferns, whose gracefully-cut 
fronds and general habit of life are too familiar to need any de- 
scription, though the reader should remember that in the tropics 
they grow into trees, among the most beautiful, though not the 
largest, that there are. Second are the Horsetails, which are stiff, 
green, rush-like plants, with terminal spore-cones, distinguished 
from the true rushes by their little leaf scales. They are no taller 
than two or three feet, and grow mostly in shoal water, or wet 
places, but sometimes on open sandy banks. Third are the Club- 
mosses, creeping, leafy, and not unlike their namesakes, the true 
Mosses, but much coarser, as the common Ground Pine well 
illustrates, or the decorative Selaginella of our greenhouses ; while 
they are further distinguished by their little terminal cone-like 
masses of spore cases. In size all three divisions of the Fern- 
plants are now greatly degenerate from a former high estate, for, 
along with others now extinct, they once grew into the trees promi- 
nent in the earlier geological periods. In color all are green from 
the chlorophyll with which they make their own food, and no 
other color occurs, save an occasional red blush in young leaves, 
and the brown of their spore-cases or stems. Their cellular anat- 
omy is well differentiated into tissues of different functions, in- 
cluding a highly-efficient system of water-carrying ducts to- 
gether with strengthening fibers; and it was the possession of this 
fibrovascular system, no doubt, which permitted these plants to 
carry their foliage high above earth upon lofty stems from deeply- 
anchored roots, thus giving the world its first forests. Their 
reproduction is by spores spread afar by the wind from the up- 
right plant, and this spore-formation alternates with fertilization 
which occurs in a way and a place not suspected by most persons. 
Thus in the true Ferns, and the process is substantially the same 
in principle in the Horsetails and Club-mosses, the little brown 
spores from the under sides of the fronds do not grow into plants 
like those which produce them, but into small (a quarter-inch in 



458 The Living Plant 

diameter) thin, filmy, green, prothallia, lying flat on the ground 
in wet places and strongly suggesting either Liverworts or Algae. 
On their under sides are well-protected egg-cells fertilized by male 
cells which swim freely through water caught under the prothal- 
lium. Then from this fertilized egg-cell arises the familiar Fern- 
plant; and we have here a very perfect example of that alternation 
of generations which is of such great botanical interest. But it is 
evident that the Fern-plants are dependent for their fertilization 
upon the presence of standing water, though this can be supplied 
by a flooding during rain-storms; and this is the reason why those 
plants are confined for the most part to shaded or moist places. 
As to their numbers, some three thousand five hundred different 
kinds are known, with doubtless not a great many more to be 
found. 

With the Fern-plants are commonly reckoned a good many 
others which do not belong there. Indeed, to most people, any 
plant with finely-cut foliage is thereby made a Fern, though many 
such plants will be found to flower at intervals. The little 
Japanese " Air-plant," graceful, feathery and deceptively Fern- 
like, is in fact an animal production, the tough horny skeleton of 
a little marine Hydroid, so naturally stained and arranged that 
not a few people declare they have witnessed it grow! 

We turn now to the place of the Fern-plants in our tree of 
descent (figure 177). All evolutionary analogy would show that 
the Moss-plants like all other groups, gave off many branches, of 
which one in particular was a brilliant success. It was the branch 
which developed a vascular system permitting the ready conduc- 
tion of water; and this freed those plants from their old ground- 
clinging habit and opened to them the upper air for the spread 
of great masses of foliage to the sun. Thus arose the Fern- 
plants, the earliest trees, which spread over the moister earth as 
its dominant vegetation, a fact which our tree represents by the 
swinging of this branch into the main trunk, displacing the Moss- 
plants. And they have persisted to the present in their own 



Groups into Which Plants Naturally Fall 459 

situations, though sadly diminished in number and size, and re- 
duced to the position of undergrowth, by the insistent and success- 
ful competition of a still higher group, the Flowering Plants. 
The three divisions they have developed are also shown by the tree. 

The Flowering Plants, called also Seed-plants, or Spermato- 
phytes. These are all the rest of the plants of the earth, com- 
prising all of the loftiest trees, practically all of the shrubs, and 
the innumerable flower-bearing herbs no matter where found, 
whether in woods, fields, waters, plains, mountains, deserts, or 
sea-shores. In shapes they are manifold, though usually dis- 
playing the characteristic differentiation into root, stem, leaf, 
flower, and fruit, the functions of which are now well known to 
the reader; but these parts may be modified multifariously in 
form, size, and combinations in adaptation to particular condi- 
tions of life. In size they range from the Redwoods, over three 
hundred feet high and thirty feet through, down to some Duck- 
weeds, hardly larger than the head of a pin. In color, since they 
make their own food, they are typically green from the presence of 
chlorophyll, though some have become parasites and lost it; but 
in some special parts, notably flowers and fruits, they have de- 
veloped well-nigh all the shades of the rainbow in adaptation to 
the accomplishment of particular functions. In their cellular 
structure they are developed beyond all other groups in special- 
ization and division of labor, which is a reason for their obvious 
and growing dominance in all situations. Their reproduction is 
chiefly through seed-formation (whence the name of the group), 
following upon the fertilization of an egg-cell in the ovule by a 
male cell brought by a pollen-tube, as already very fully described 
in our chapter on Reproduction. 

This fertilization arrangement, whereby a male cell is carried 
by a tube from a pollen grain to an egg-cell borne high on a plant, 
seems at first sight to possess nothing in common with that in the 
Fern-plants, where the male cell swims freely to the egg-cell 
through water caught under a prothallium pressed close to the 



460 The Living Plant 

ground. But in fact there is every gradation between them, and 
one answers morphologically to the other in a manner most 
striking and satisfactory, though it is not any part of my business 
at present to explain the matter more fully. But there is one 
thing about this pollen-tube arrangement that is of greatest 
evolutionary importance, viz., it has rendered these plants in- 
dependent of standing water and a prothallium on the ground for 
their fertilization, and has thus freed them from the restriction 
which limits the range of the Fern-plants. Hence the Flowering 
Plants are able to extend over places too dry for the Fern-plants, 
and indeed over all parts of the earth where plant-life is a possi- 
bility at all; and not only that, but through virtue of their higher 
organization in other respects they are able to compete with the 
lower groups, the Undergrowth plants, the Carpet plants, the 
Parasitic plants, and the Water plants, in their own peculiar 
situations, of which they are slowly but surely taking possession 
in the course of their evolution. And the best evidence of their 
success is found in their numbers, for they have been able to 
develop no less than some one hundred and thirty-three thousand 
distinct species already known and named, many more, it will 
be noted, than of all the other groups put together. 

The Flowering Plants include two very distinct groups. First 
are the GymnospermSj Pines, Spruces, Firs, and that sort, 
which are trees and tall shrubs without any flowers, and bearing 
their seeds naked on the branches, or partly covered by cone- 
scales; and they are almost wholly wind-disseminated and wind- 
pollinated. Second are the Angiosperms, with their seeds en- 
closed always in an ovary which is part of a flower. Some of 
them, the Oaks, Chestnuts, Beeches, Elms, Birches, Alders, and 
such kinds, are trees or tall shrubs, wind-pollinated (and there- 
fore without showiness in the flowers) and wind-disseminated. 
The remainder fall* into two sub-groups, Dicotyledonous or Ex- 
ogenous Plants, which appear to occupy the main line of advance, 
and Monocotyledonous or Endogenous Plants, which seem to 



Groups into Which Plants Naturally Fall 461 

have been separated from the Dicotyledons through an early 
partial return to a water habit. Both of these sub-groups are 
distinguished for the most part by insect-pollination, with its 
correlated floral showiness; and so much more effective and 
economical is this insect-pollination than wind-pollination that 
the Flowering trees, Locusts, Magnolias, and most Fruit trees, 
are slowly driving the wind-pollinated kinds from the earth. 
This insect-pollination, moreover, with which naturally goes 
animal-dissemination, renders the plants independent of exposure 
to winds for both pollination and dissemination, and hence capable 
of growing in all kinds of retired and lowly situations. Therefore 
there exist not only Flowering shrubs, which can grow as under- 
growth in successful competition with the Fern-plants, but also 
Flowering herbs, which can grow in all sorts of places, even in 
competition with the carpeting Moss-plants, with the Water- 
plants, and with the Parasites. 

We turn now to the place of the Seed-plants in our tree of 
descent (figure 177). Among the branches produced by the 
Fern-plants must have been one with a wind-carried, tube- 
producing, pollen-grain, a discovery, or invention, which ren- 
dered its possessors independent of the standing water needed by 
the Fern-plants for fertilization, thus enabling them to range far 
more freely over the earth. Such was the origin of the Seed- 
plants, which swung into the main line of dominance, where they 
persist to this day. The first kinds were undoubtedly trees, 
Gymnospcrms and wind-pollinated Angiosperms, whose exact 
relations to one another are still very uncertain; but from the 
latter originated the insect-pollinated kinds, first trees, then 
shrubs, then herbs. These latter possess all of the advantages of 
the lower groups in addition to their own, are the heirs of all 
the ages in fact; and their higher organization is permitting them 
to do precisely the same thing that the higher races of men are, 
to take possession of the earth to the suppression and extinction 
of the lower races. 



462 The Living Plant 

Such are the five primary groups, sometimes called Classes, of 
Plants. Each is divided into sub-groups called Orders, and those 
again into others called Families, and those again into others 
called Genera, and those into Species. It is theoretically possible 
to follow out the branches of our genealogical tree through smaller 
and smaller ramifications to the ultimate tips, representing the 
species, of which there would be some two hundred and fifty 
thousand ; and the construction of such a tree is the aim of every 
student of classification. It is, however, no part of our present 
business to follow this matter any farther, for, while the primary 
groups are distinguished very largely by differences of habit, this 
becomes less and less true with the groups that are smaller, and 
hardly at all with the species, which are mostly marked off from 
one another by characters having little connection with adapta- 
tion. 

The Flowering Plants are the highest yet developed within the 
Plant Kingdom. Are there then no higher possibilities in plant 
evolution? So far as concerns any new field for them to expand 
in, there seems to be none, unless they follow the example of man, 
and take to free flight in the air. But the world is not yet finished, 
nor are alt the possibilities of variational experimentation ex- 
hausted; and until such times come, evolution is not likely to 
cease. 

There remains one other aspect of classification to be men- 
tioned before this chapter can be finished. Although the large 
genealogical groups we have been considering happen to be dis- 
tinguished pretty well from one another in habit, and thus con- 
stitute also ecological groups, the correspondence between gen- 
ealogy and ecology is by no means exact. Examples, indeed, of 
the ecological intrusion of the genealogical groups into one an- 
other have been given in the preceding pages; and further study 
only serves to increase the number of such cases. Every group 
is striving to expand to its utmost, and whenever it can find an 
unoccupied crevice in the territory of another, it is not deterred 



Groups into Which Plants Naturally Fall 463 



Strand-plants 
(FlalophyteH) 



Air Plants, 
/ Climbers, Ac. 
(Epiphytes, &c.) 



= JET -=-"~-==~=-^r== r "~ -=^~ = =- =--=1 Saprophytes 




FIG. 178. A diagram showing the mutual interrelations of the genealogical and ecological 
groups. The widths of the connecting bars show the approximate number of species 
involved. 



464 The Living Plant 

by any genealogical courtesies from expanding to fill it. The re- 
sult is this, that kinds of plants genealogically related have come 
to acquire very different habits, and hence to belong to very 
different ecological groups, while the different ecological groups 
include many kinds having the most different genealogical rela- 
tionships, a matter which is brought out diagrammatically in the 
accompanying figure (figure 178). It is with plants as with men, 
who may be grouped by their blood relationships into families or 
clans on the one hand, or according to their occupations into 
trades, businesses, or professions on the other. Sometimes the 
two arrangements overlap, especially among primitive peoples, 
but often they do not, particularly in the higher civilizations. 
These ecological groups of plants have been characterized more or 
less fully in the preceding pages, and need only be summarized 
very briefly at this place. 

A SYNOPSIS OF THE ECOLOGICAL GROUPS OF PLANTS 

I. INDEPENDENT PLANTS, or AUTOPITYTES, the highest and most 
distinctive plants, making their own food by aid of chlorophyll, and in- 
cluding : 

1. NORMAL PLANTS, or MESOPHYTES, living rooted in aerated soil sup- 
plying enough moisture to permit a wide spread of leaves and stems; 
mainly Flowering plants, but with many Fern-plants and Moss- 
plants, commonly massed together into forests which exhibit a canopy 
of trees, an undergrowth of shrubs, and a carpet of herbs. 

Furthermore, some kinds of Mesophytes are so strongly adapted 
to some particular condition of life as to rank as separate groups, viz., 
Air plants, or EPIPHYTES, including members of all the genealogical 
groups, growing supported upon other plants, and highly adapted 
to that peculiar habit: CLIMBERS, mostly Flowering plants, whose very 
slender stems, lean, cling or twine by aid of others up to the light: 
TRAILERS, of all groups, which keep flat on the ground as a part of the 
carpet: INSECTIVOROUS Plants, wholly Flowering plants, which sup- 
plement the scantness of soil nitrogen in the places where they live by 
capturing and digesting insects through aid of remarkable adaptations: 
MYRMECOPHILOUS Plants, Flowering plants of the Tropics, supposed 
to attract ants for protection against other insect enemies, but of doubt- 
ful ecological status at present. 

2. WATER PLANTS, or HYDROPHYTES, living largely immersed in water 
from which they take their minerals and gases, and therefore mostly 
soft-bodied and finely divided; mainly Algae, but including some Moss- 
plants, Fern-plants, and Flowering plants. 



Groups into Which Plants Naturally Fall 465 

3. STRAND PLANTS, or HALOPHYTES, living along the margin of salt 
water, and therefore condensed and otherwise adapted to the difficult 
absorption thereof; a few Flowering plants only. 

4. DESERT PLANTS, or XEROPHYTES, living in places excessively dry, 
and therefore condensed and protected for water conservation; mainly 
Flowering plants, with a few Lichens. 

II. DEPENDENT PLANTS, sometimes called HYSTEROPHYTES, in- 
cluding PARASITES and SAPROPHYTES, which take their food from 
other organisms, cither living or dead, and lack chlorophyll and leaves; 
mainly Fungi, but including some Flowering plants. 

Finally, there is one more way in which plants are classified 
ecologically. When considered en masse, plants constitute vege- 
tation, and vegetation can be classified. A mass of vegetation 
which gives a distinctive aspect to a country, such as a Pine 
forest, or a natural meadow, is called a Formation; any group of 
plants commonly occurring together therein is called an Associa- 
tion; while the word Society is somewhat loosely used for any kind 
of vegetation group. This subject is one very much studied at 
present, and will ultimately give us a vivid method of describing 
causally the vegetation of any country. 



INDEX 



Figures in heavy-faced type indicate illustrations 



Abnormal growths, 357 

Absorption, water, 165; system, 168, 268; 

machinery, 169; by roots, 172; cortex 

to ducts, 174; of minerals, 189; of 

gases, 190; of organic substances, 193; 

of salt water, 268 
Acacias, 70 
Acanthus, 385 
Accident in growth, 369 
Acclimatization, 254 
Accumulation of characters, 409 
Acids, 117; chemistry, 118 
Adaptation, nature, vi, vii, 11, 12, 47, 60, 

424 

Adjustivc vs. adaptive structures, 254 
Adjustment of plants to light, 224; to 

moisture, 239; to chemical substances, 

241; to touch, 242; to gravitation, 245; 

to minor influences, 251; in growth, 

369 
Aeration; system, 190, 191, 192, 206, 

271; in water-plants, 194; of soils, 85 
Aerial roots, 68, 382 
Aerotropism, 241, 242 
Aging of plants, 163 
Agriculture, 3 
Albumins, 127 
Alcohol, formation, 97, 98 
Algae, of hot springs, 265; fertilization, 

304; described, 445 
Algal paper, 377 
Alkaloids, 103, 106, 125, 274 
Alpine plants, 321 
Alterability of individuals, 413 
Alternation of generations, 455 
Amides, 106, 120, 125 
Amoeba, 380 
Amygdalin, 119, 120 
Anatomy, defined, 2 



Anchor roots, 196, 358 

Angiosperms, 460, 461 

Animals, protection against, 274; pol- 
lination by, 309; dissemination by, 
394, 461 

Annuals, 366 

Annual rings, 365 

Anther, 281 

Anthocyan, 39, 119 

Antibodies, 254 

Ants, and flowers, 325; and dissemina- 
tion, 398 

Aphorisms, of Bacon, 9 

Applications of science, 4, 5 

Aristolochia, fertilization, 311 

Ascent of sap, 213 

Asparagin, 120 

Association, 465 

Atmospheric pressure, 215 

Autophytes, 464 

Autumn coloration, described and ex- 
plained, 40, Plates II, III; significance, 
43; causes influencing, 45, 148 

Autumn, vegetation in ; 368 

Auxograph, 329, 330 

Bacteria, 4, 102, 122, 262, 273, 281, 391, 
451; chcmosynthetic, 448; dissemina- 
tion, 452 

Bacteriology, 3 

Barometric pressure and growth, 336 

Basal food, 106 

Bayberry wax, 118 

Bezoars, 377 

Biennials, explained, 366 

Biology, defined, 3 

Biotypes, 420, 423 

Birds, in cross pollination, 323; in dis- 
semination, 395, 397 



467 



4 68 



Index 



Birdseye Maple, 372, 373 

Birth-rate in man, 302 

Black-knots, 451 

Bladders, on fruits, 388 

Blades, of leaves, 70 

Blights, 273, 451 

Blue-green Algir, 380, 448 

Body plasm, 415 

Boston Ivy, 13, 232 

Botanical study, 1 ; education, 3 

Botany, defined, 2 

Bracts, 70 

Bracket-fungi, 451, 452 

Branching, explained, 49 

Bread raising, 98 

Breathing, 85, 191 

Brown Alga 1 , 447 

Brown in leaves, 44 

Browning of evergreens, 202 

Bryophytes, 454 

Buds; originate* stems, 61; coverings, 67; 

protection of, 277; development, 359, 

360; position, 361 
Bud sports, 437 
Bulblets, 279 
Burbank, 443 
Burdock, 396 
Bursting of pavements, 79 
Butcher's Broom, 71 

Cactus, 68, 70, 212, 264, 269, 277, 
396 

Caffeine, 125 

Cambium, 155, 221, 362 

Camphor, 117 

Cane sugar, chemistry, 27, 108 

Capillarity, 169; phenomena, 179; ex- 
plained, 179; diagram, 179, 180; in 
plant life, 181, 215 

Carbohydrates, 106, 107; derivatives, 
106, 116 

Carbon dioxide, in atmosphere, 29; in 
photosynthesis, 30, 31, 35, 48; in 
respiration, 81 

Carbon izat ion , 113 

Cardinal points, 333 

Carpet plants, 454, 460 

Caruncle, 398 



Cell division, 284, 287, 341, 355 

Cells, described, 20, 160; sugar holding, 
107; conventionalized, 161; shapes, 
153, 154; discovery, 158; named, 158; 
why exist, 161; sizes, 161; enlarge- 
ment, 342, 343 

Cellulose, described, 111; chemistry, 113; 
modifications of, 113; alteration to 
coal, 113, 157 

Cell wall, 150, 151; shapes, 153; thick- 
ness, 166; composition, 157 

Chemosynthetic Bacteria, 448, 449 

Chemotropisrn, 241 

Chimaeras, 351 

Chlorophyll; distribution, 17, 18; flu- 
orescence, 19; properties, 19; insta- 
bility, 19; grains, 21, 22, 110, Plate I; 
and light, 32, 34; in photosynthesis, 35; 
why green, 37; composition, 117; origin 
in time, 450 

Chloroplastids, 160 

Chondriosomes, 159 

Chromatin, 284 

Chromosomes, in division, 77, 284, 287; 
composition, 129, 160, 162 

Chrysanthemum, improvement of, 42(5, 
427 

Cion, 349 

Circulation of substances in Nature, 133 

Circumnutatiori, 77, 346, 348 

Cladophora, 447 

Classes of plants, 462 

Classification, 2, 404, 445 

Cleistogamous flowers, 316, 318 

Clematis seed, 389 

Clerodendron flowers, 316 

Climbers, 242, 464 

Climbing organs, 72 

Clinostat, 238, 239 

Close pollination, 438 

Clover, fruits, 387 

Club-moss, 456, 457 

Clusters of flowers, explained, 321 

Coal, formation, 114 

Cobcea scandens, 326 

Cocaine, 125 

Cockscomb, 373 

Cocoanut, dissemination, 394 



Index 



469 



Color 17, 37; in seaweeds, 38, 446; in 
foliage plants, 38; in spring vegeta- 
tion, 39; screens, 39; in autumn leaves, 
40; Plates II, III; in cross pollination, 
308, 310, 319, 320; contrasts, 318; 
variegation, 321; changes, 321; of 
fruits, 398; in Fungi, 452 

Compass Plant, 232, 264 

Correlation in growth, 372 

Combustion, 89, 114 

Composite conceptions, 7, 9 

Compounding of leaves, 59 

Compromises in structure, 223 

Conduction, 145; system, 168 

Cone shapes of trees, 56, 58, 260 

Consciousness, 255 

Continuity of protoplasm, 157, 158 

Conventional constants, explained, 9; 
26, 28, 31 

Copper beeches, 38, 436 

Corallines, 447 

Cork, 155 

Corn, silk, 307 

Cortex, 222 

Cotton seed, 389 

Cotyledons, 354 

Cross fertilization, 294; advantage of, 
295; in water plants, 303 

Crossing, 437 

Cross pollination, 303; vs. cross fertili- 
zation, 303; by water, 304; by wind, 
305; by insects, 309; books on, 314; 
arrangements to ensure, 315; by birds, 
323; by snails, 324, 438 

Crystals, 134, 274 

Cultivation, 429, 442 

Cup leaves, 375 

Curbstones lifted, 78 

Cutin, 113, 157 

Cut-leaf forms, 436 

Cycles, in life, 163; in growth, 352 

Cytase, 128 

Cytology, defined, 2 

Cytoplasm, 150, 159 

Dandelion seed, 389 

Darkness, and plants, 91, 334 

Darwinr letter, 6; 11, 12, 234, 266, 294, 



314, 346, 347, 349, 406, 409; biography, 
411, 417, 419, 423 

Date seed, 112, 156 

Death; 144, 163; cause in trees, 367 

Decay, described, 102 

Deciduous trees, shape, 51, 261, 262 

Denatured alcohol, 99 

Desert plants, characters, 203, 264, 465 

Development, 327, 340, 353 

De Vries, 418, 420, 423 

Dextrose, 27; chemistry, 107 

Diagrams, nature, 9 

Diastase, 110, 128 

Diatoms, 380, 420, 446 

Dichogamy, 315 

Dicotyledons, 460 

Diffusion, nature of, 175; diagram of, 
176; of gases, 193, 206, 218 

Digestion, 110, 218 

Dimorphism, 316, 317 

Diseases, 102 

Dissemination, 116, 118, 279, 378; rea- 
sons for, 378; methods, 379; locomo- 
tion, 379; vs. cross pollination, 379; 
by growth, 381; by shortening roots, 
382; by projection, 383; by hygro- 
scopic movements, 385; by gravita- 
tion, 387; by winds, 387; by reduction 
in size, 390; by water, 393; by animals, 
394; by man, 399; minor adaptations, 
401; books on, 401 

Distillation, 99 

Diversity of plants, 16 

Division, 145, 280 

Dixon and Joly, 216 

Dodder, 453 

Dominance, 296 

Double fertilization, 300, 356 

Double flowers, 375, 436 

Double fruits, 273 

Drainage, 87, 201 

Drains, filled by roots, 241 

"Drawing" of plants, 334, 431 

Dreams, 357 

Drowning of roots, 87 

Dry-blasting of plants, 202 

Dryness, protective, 266; protection 
against, 267 



470 



Index 



Dry weights, 344 
Duckweeds, 393, 448, 459 
Ducts, 155, 213, 221 
Dulse, 447 
Dust, and spores, 391 

Ecology, defined, 3; vs. morphology, 73 

Ecological groups, 403, 464 

Economic Botany, 3 

Edible fruits, 397 

Eel grasses, 195, 305, 448 

Egg cell, 281 

Electricity and growth, 336 

Electrolysis, 35 

Electrotropism, 251 

Elementary species, 420 

Emaciation, 91 

Embryo, 354 

Embryology, defined, 2, 405 

Embryo sac, 281 

Endogenous, vs. exogenous, 364; 461 

Endosperm, 300, 354 

Energy in plants, 79; source, 80, po- 
tential vs. kinetic, 93 

Enzymes, 106, 126; principal kinds, 
128; catalyzers, 129; in digestion, 
217 

Eoprotista, 448 

Epidermis, 155, 222; of desert plants, 
269, 270 

Epiphytes, 464 

Erythrophyll, described, 39; uses, 39; 
in autumn leaves, 42; 119 

Essential oils, 117 

Ether and growth, 336 

Ethereal oils, 117 

Eucalyptus, 201 

Euphorbia splendens, 68 

Evergreens, shape, 68, 259 

Evolution, 403; vs. special creation, 403; 
evidence, 404; explanations, 404; a 
scientific question, 405; by natural 
selection, 406; diagram, 410; and 
Darwin, 411; and Lamarck, 412; by 
transmission acquired characters, 413; 
and Weismann, 415; epochs, 417; and 
de Vries, 418; by mutation, 418; and 
Mendel, 422; new indications, 423; 



under experiment, 425; vs. improve- 
ment, 429 

Excretion, of gases, 211; of minerals, 
212; of root-poisons, 212; of nectar, 
212; of nitrogen, 125 

Exogenous vs. endogenous, 364; 460 

Experiment, use, 22, 31 

Experiment greenhouse, 8 

Explosion, of fruits, 384; of stamens, 323 

Experimental evolution, 418, 425 

Extra-floral nectar, 212 

Fairy Rings, 381, 382 

Fasciations, 373, 374 

Female, meaning of, 291 

Fermentation, described, 97; economics, 
98; chemistry, 99; object, 100; relation 
to respiration, 100, 101; by-products, 
101 

Ferments, 128 

Ferns, 457; fertilization of, 241, 283, 289, 
290; 304 

Fern-plants, described, 456 

Fertilization, described, 281, 283, 285, 
286, 437, 459 

Fibres, 221, 222 

Fibrovascular bundles, 221; 363, 364, 
365; tissues, 457, 458 

Fission, 280 

Flesh-formers, 106, 126 

Flotage of seeds, 393 

Flowering Plants, described, 459 

Flowers, morphology, 69; turn to light, 
235; 236; protection of, 276; geotro- 
pism, 249; described, 281, 282; pecul- 
iarities explained, 310: defined, 310; 
correlations with insects, 317; essential 
parts, 362; green, 375 

Flowers of tan, 141 

Fluctuating variations, 423 

Foliage plants, 38, 428 

Food, function, 94; a storage battery, 96; 
reservoirs, 68; 106, 107 

Forestry, 3 

Fortuity of variation, 433 

Fossils, 404 

Freaks, 373, 436 

Frost, and autumn colors, 45 



Index 



47* 



Frost weeds, 211 

Fructose, 27; 108 

Fruit Jellies, 115 

Fruits, turn to light, 235; from light, 
226, 235; protection of, 276; 378; func- 
tion, 391 ; vs. seed, 391 

Fruit sugar, chemistry, 27, 107 

Fungi, described, 450 

Galls, 370 

Gases, absorption, 190 

Gelatine, 115 

Genealogical, trees, 448, 449; groups, 463 

Generalizations, 7 

Generalized drawings, nature, 9 

Generalized knowledge, 9 

Genotypes, 298, 423 

Geographical distribution, 404 

Geotropism, 245; of stems, 245; of roots, 

228, 246; of leaves, 248; of flowers, 248; 

of fruits, 250; transverse, 246, 250; 

lateral, 250; correlation of, 373 
Germination, 358 
Germ cells, 415, 421 
Germ plasm, 415 
Gibson, 315 
Globulins, 127 
Glucosides, 119, 274 
Glutelins, 127 
Gradations, 404 
Grafting, 293, 349; effects scion on stock, 

350; 437 

Graft hybrids, 351 
Grand period of growth, 337 
Grape sugar, 27; 107 
Graphs; of transpiration, 203; of growth, 

330, 337 

Grappling Plant, 395, 396 
Gravitation, and plants, 247; as stimulus, 

358, 359; in dissemination, 387 
Gray, Asa, 0, 12, 71, 315 
Grayness of vegetation, 263 
Green Alga*, 446, 454 
Green color in plants, 17 
Green-manuring, 124 
Green Roses, 376 
Grew, Nehemiah, 158, 220 
Growth, 145; 327; operations, 327; de- 



velopment, 327, 340; enlargement, 327, 
328; maturation, 327, 345; measure? 
ment, 328; graphs of, 330, 331, 337; 
and temperature, 332, 333; and light, 
334, 335; and transpiration, 335; -and 
humidity, 336; and electricity, 336; 
and poisons, 336; and ether, 336; and 
barometric pressure, 336; of leaves^ 
roots, stems, 338, 339, 340; shortening, 
338; minor phenomena, 345; move- 
ments, 346; cell division, 341; water 
in, 343; and osmotic pressure, -342; 
structural phenomena, 352; cycles, 
352; egg-cell to embryo, 363; germinar 
tion, 358; seedling, 369; to adult, 361 : ; 
embryonic vs. permanent tissue, 362; 
secondary growth, 362; system, 303; 
and age, 366; seasonal cycles, 367; 
disturbance, 369; monstrous, 373; dis- 
semination by, 381 

Guard cells, action, 207, 208 

Gum arabic, 114 

Gums, chemistry, 114 

Guttation, 210 

Gymnosperms, 460, 461 

Ilabenaria, fertilization, 312, 313 

Hairs, 389 

Hales, Stephen, 5, 223 

Halophytes, 464 

Heat, from respiration, 90; protection 

against, 265 
Heliotropism, 227 

Hemispherical shape, explained, 50, 51 
Herbaria, 2 
Heredity, 408 
Hildebrand, 401 
Hooke, Robert, 158 
Hooks on seeds, and fruits, 70, 395, 396 
Horsetails, 457 
Horticulture, 3 

Hot springs, plants in, 265 , 

House plants, how determined, 201 
Humidity and growth, 326 : 
Humus, 122 
Huxley, 12, 141, 412 
Hybridization, nature, 437, 439; practice, 

441 



472 



Index 



Hybrid lilies, 440 
Hybrids, characters, 440 
Hydrolysis, 217 
Hydrophytes, 464 
Hydrotropism, 239, 240 
Hygrometers and bygroscopes, 189 
Hygroscopic movements, 385 
Hygroscopicity, explained, 188; move- 
ments of, 189 
Hypocotyl, 354 
Hypothesis, use of, 32 
Hysterophytes, 465 

Imbibition, explained, 178; 187, 215 

Immunity, 273 

Improvement, illustration, 428; hypo- 
thetical case, 434 

Inbreeding, 438 

Incidental phenomena, 347 

Indian Pipe, 453 

Indigo, 119 

Initial cell, 353 

Insanity, 357 

Insectivorous plants, 69, 194, 245, 
464 

Insect pollination, 309, 461 

Insect traps, 69 

Insects, and flowers, 317, 323; resem- 
bling seeds, 399 

Intelligence in Man, 14; in Nature, 14 

Intercellular system, 190 

Internodes, explained, 61 

Invertase, 129 

Iodine test for starch, 23, 109 

Iris, flower, 316 

Irish Moss, 447 

Irritability, 145, 148; nature of, 224; 
227; elements in, 227; nature of re- 
sponses, 229; stimulus, 229; localized 
perception, 234; motor mechanism, 
228; vs. heredity, 237; in plant im- 
provement, 431 

Isolation of biotypes, 432 

Ivory Palin, 112, 156; cellulose of, 
113 

Japanese Air-plant, 458 
Juvenescence, 301 



Kelps, 447 
Kerner, 314, 401 
Knuth, 314 

Laboratory methods, value of, 138 

Lamarck, biography, 412 

Lathyrus Aphaca, 68 

Leaf fall, 40 

Leaves, anatomy, 19, 21, Plate I; weight, 
25; why exist, 50; characteristics, 51; 
parts, 52 ; typical, 53 ; variety of shapes, 
53, 64; typical shapes, 55, 66; con- 
ventionalizations, 67; terminology of 
shapes, 58; emarginations, 59; com- 
pounding, 59; lobing, 60; gas move- 
ments in, 84; sizes, 65, 258; compound- 
ing, 259; thickness, 259 

Leaf mosaics, 233; 235 

Lecithins, 117 

Lenticels, 191 

Leucoplastids, 160 

Lichens, 451 

Life, nature, viii, 13, 14; characteristics 
of, 95; in relation to carbon, 96; energy 
changes, 104, 134; two elements in, 
144; origin of, 96, 149; cycles, 163; 
rejuvenation, 163 

Life in relation to carbon, 96; all earth 
can support, 104 

Life-plants, 392 

Light, nature, 32; use in photosynthesis, 
34, 48, 50; on autumn color, 148; ad- 
justment to, 224, 226, 235; protec- 
tion against, 256, 261; and growth, 
334 

Light screens, 39, 43, 262, 455 

Lignin, 113, 157 

Linaria Cymbalaria, 237 

Linden, fruits, 389 

Linnaca, 326 

Linnaeus, quoted, 11 

Lipase, 128 

Liquors, origin, 98 

Liverworts, 393, 454 

Living Oats, 385 

Living plant, an energy station, 198 

Living protoplasm, chemistry, 106, 13 

Locomotion of plants, 304, 379 



Index 



473 



Long Moss, 456 
Lotus, 394 
Lubbock, 315 

Machinery of photosynthesis, 36, 37; of 
respiration, 89; of absorption, 169; 356 

Macrocystis, 445 

Madder, 119 

Malaria, 201 

Male, meaning of, 291 

Malpighi, 158 

Manuals, 2 

Manufacture of sugar, 35 

Maple, sugar, 108; fruits, 388 

Margins of leaves, 59 

Maturation, 327, 345 

Mechanical causation, vi, 65; responses, 
245 

Mechanism, viii, 13 

Medullary rays, 222, 365 

Membranes, nature of, 173, 176, dia- 
grams of, 177 

Mendel, 295; his Law, 295; diagram, 297; 
work, 417; in evolution, 422, 439 

Meristem, 340, 362 

Mesophytes, 464 

Metabolism, 105, 145 

Methods of study, 1 

Micella?, explained, 146, 176; diagrams 
of, 177 

Microbes, 451 

Micropyle, 281 

Mildews, 273, 451 

Mimicry, 275, 399 

Mind, in research, 10 

Minerals, in photosynthesis, 48; in plants, 
134, 136; absorption by plants, 189; 
as excretions, 212 

Mistletoe, 397, 453 

Mitochondria, 159 

Mobility, 145 

Mohl, H. von, 156, 158, 159 

Molds, 273, 451 

Molecular vs. molar forces, 194 

Molecules, 175 

Monocotyledons, 461 

Monstrosities, 357, 373, 375, 436 

Morphine, 125 



Morphology, defined, 2, 67; vs. ecology, 

73; diagram, 74; 404 
Mosses, 454 
Moss-plants, 454 
Moth, pollinator, 314 
Mucilage; 157; on seeds, 396, 400 
Mucilaginous modification, 113 
Muehlcnbeckia, 71 
MUller, 314 
Multiplication, 301 
Mummy seeds, 355 
Muscarine, 125 
Mushrooms, 451 
Mutants, 419, 432 
Mutation, explained, 419 
Mycelia, 381, 452 

Mycorhiza; absorption by, 193; 451 
Myrmecophila, 464 

Nastic movements, 252 

Natural History of Plants, 3 

Natural Selection, 257; explained, 406 

Navel oranges, 375, 436 

Nectar, use, 309; 322; glands, 310 

Nectaries, extra-floral, 325 

Nectarines, 436 

Nelumbium, 395 

Nepenthes, 69 

New organs, origin, 73 

Nicotine, 125 

Nitrification of soils, 121, 123 

Nitrifying Bacteria, 122, 123 

Nitrogen; assimilates, 106, 120; in at- 
mosphere, 120; in plants, 120; ex- 
cretion, 125; activity, 126 

Nodes, explained, 61, 381 

Nodules of Leguminosic, 123, 124 

Non-adaptive features, 13 

Non-green plants, 17 

Nucleo-protcins, 127 

Nucleolus, 150 

Nucleus, structure, 142; 150 

Oak tree, 62 

Observation, use of, 18, 31 

Odor, in flowers, 321; vs. color, 322 

(Edema, 186 

(Enothera Lamarckiana, 418, 419, 423 



474 



Index 



Oil gland, 118 

Oils, 274 

Optical sections, 19 

Orchid seeds, 389 

Origin of protoplasm, 148 

Osmoscope, 171 

Osmosis, experiment, 170; permeable 
and semi-permeable membranes, 173; 
pressures, 182; explained, 184; dia- 
gram of, 184; phenomena, 185, 18(5 

Osmotic absorption, 17)3, 268; turges- 
cence, 52; pressures, amount, 182; ex- 
planations, 183; phenomena, 185 

Ovary, 281 

Overproduction, 407 

Ovules, 281, 283; changed to leaves, 375 

Oxygen, in photosynthesis, 30, 31; in 
respiration, 82 

Pangenesis, 415, 417 

Parasites, 17; and growth, 370; 450, 4(55 

Parthenogenesis, 299 

Partridge Berry, 317 

Pasteur, 148 

Pathology, defined, 3 

Pavements burst, 78, 79 

Pectins, 115 

Peptones, 127 

Perception, 229 

Perennials, explained, 366 

Personification of Nature, 326 

Petioles, 52, 70; function, 258 

Pfeffer's cell, 182 

Phsenotypcs, 423 

Pharmacology, 3 

Philosophy of nature, 403 

Phosphoproteins, 127 

Phosphorus in plants, 126, 128 

Photonasty, 252 

Photosynthate, defined, 24; quantities, 
25 

Photosynthesis; defined, 24; product, 24; 
27; materials used, 28; carbon dioxide, 
29; water, 29; oxygen, 30; equation, 
31; quantities, 25, 26, 31; and chlo- 
rophyll, 32; and light, 32, 34; as manu- 
facture, 35; visualization, 36; machin- 
ery, 36, 37, Plate I; adaptations 



thereto, 47, 50; the four essentials, 48; 
effects on plant form, 48 

Photosynthetic, equation, 31; tree, 51; 
sugar, 106; its fate, 132 

Phototropism, explained, 225, 226; ex- 
periment, 226; negative and trans- 
verse, 227, 232; of leaves, 225; of steins, 
225; of roots, 226; of flowers, 234; of 
fruits, 235 

Phyllotaxy, described, 62; systems, 62, 
63; explanation, 64, 360 

Physiological method, 203 

Physiology, defined, 2 

Pineapple, fasciated, 374 

Pistils, open, 375 

Pitcher leaves, 69, 375 

Pitcher Plants, 68, 69, 194 

Pith, 222 

Plant breeding, defined, 3; 273; 404, 426; 
products, 426; by selection, 429; pres- 
ervation of sports, 435; crossing and 
hybridization, 437; cultivation, 442; 
theory, 443 

Plant fats, 116 

Plant Industry, defined, 3 

Plant improvement, methods, 428 

Plant oils, 116; in seeds, 116 

Plants, kinds of, 1 

Plasomen, 146 

Plastids, 110, 150, 160 

Pleasures of Botany, 5, 6, 7, 46; of 
science, 204, 217 

Pleurococcus, 446 

Plumes, 389 

Plumule, 354 

Poison Ivy, 117, 275 

Poisons, 106, 125; and growth, 336 

Pollen, damaged by water, 272; 281; 
male cell, 282, 284; grains, described, 
306; protection of, 324; projected, 
323; tube, guided, 241; growth, 282, 
291 

Pollination, 282 

Poppy pods, 387 

Potash in plants, 135, 136 

Potential vs. kinetic energy, 93 

Potted plants, care of, 87 

Pressure by roots, 78 



Index 



475 



Projection of seeds, 383, 384, 385 

Prolamins, 127 

Proliferation, 374, 375 

Propulsion, of water up stems, 215 

Protease, 128 

Protection, 256; adaptations, 257; 
against winds, 257; against light, 261; 
against weight of snow and ice, 259; 
against heat, 265; against dryness, 
267; against too much water, 271; 
against parasitic plants, 273; against 
animals, 274 

Proteins, 106, 126; chemistry, 126; 
grains, 127 

Proteoses, 127 

Prothallia, 458 

Protista, 448 

Protoplasm; in cells, 22; a real sub- 
stance, 138; appearance, 139, 140, 143; 
146; streaming, 140, 141; texture, 141; 
pictures of, 142; chemistry of, 142, 
144, 147; lability of, 143; par excel- 
lence, 143; regulatory power in, 144; 
physiological properties of, 145; af- 
fected by external conditions, 147; 
origin of, 148; organization of, 149; 
identity in animals and plants, 153, 
continuity of, 157; named, 159; why 
separated into cells, 161; rejuvenation 
of, 163; vitality suspended, 164; pro- 
tection of, 257 

Protoplasmic streaming, 77 

Ptcridophytes, 456 

Ptomaines, 103 

Puff-balls, 451 

Pulp of fruits, 398 

Purity of germ cells, 298 

Purposefulness, vii, 11, 12 

Quartered oak, 365 
Quinine, 125 

Radial structure, 49 

Radium, effects on plants, 251 

Rain, protection, 324 

Ratification, 101 

Reason, 255 

Red Alga?, 289, 447 



Red Snow, 447 

Redness of spring vegetation, 263 

Reduction; in size, 390; division, 285, 
298, 422; of surface, 271 

Reflex action, 255 

Regulatory power in life, 144 

Regulators of metabolism, 106, 128 

Reindeer Moss, 456 

Rejuvenescence, 163, 367 

Reproduction, 278; asexual described, 
279; sexual described, 281; in relation 
to sex, 286; structures, 286; in relation 
to characters, 293; Mendelian basis, 
295; balance with vegetation, 301 

Resins, 274 

Respiration; balance, 31; nature, 80; 
experiment, 81, 82; release of carbon 
dioxide, 81; absorption of oxygen, 82; 
vs. photosynthesis, 83, 94, 95; quan- 
tities, 83; vitiates air, 84; balance with 
photosynthesis, 84, 85; vs. breathing, 
85; chemistry, 88; equation, 88; ob- 
ject, 88; vs. combustion, 89, 90; ma- 
chinery, 89; release of heat, 90; loss 
of weight, 91; source of energy, 92; re- 
lation to fermentation, 100; anaerobic, 
102; intramolecular, 102; relation to 
decay, 102; relation to disease, 102; 
and cell size, 162 ; and movements, 228 

Respiratory equation, 88 

Resting state, 357 

Resurrection plant, 392 

Reversions, 375 

Rhcotropism, 251 

Rockwceds, 289, 447 

Rogues, 438 

Roots; characteristics, 66; as foliage, 71; 
drown, 87; excretions, 125; structure, 
165, 195; hairs, 165, 166; anatomy, 
167; cap, 166; growing point, 166; 
absorption by, 172; adaptations, 196; 
morphological modifications, 197; ori- 
gin of, 196; poisons, 212; pressures ex- 
erted, 78, 214; prop, 258; shortening, 
382. 

Rose of Jericho, 392 

Rots, 273, 451 

Rubber, 119 



Index 



Rudimentary structures, 405 
Runners, 381 
Rusts, 273, 451 

Sachs, 32, 215 

Salt marshes, 271 

Salvia, fertilization, 312 

Sand Box, 384 

Sap-cavities, 150, 161; growth, 342 

Saprophytes, 17, 450, 465 

Sargassum, 447 

Scientific procedure, 31 

Scion, or cion, 349 

Scorpiurus, 399 

Sea-mosses, 447 

Secondary sexual characters, 288, 291 

Secretions, 106, 116, 220 

Sedentary habit, 49, 378 

Seedling, described, 359, 361 

Seed-plants, described, 459 

Seeds, 354; vitality, 355; from mummies, 
355; protection of, 276; animal car- 
riage, 394; dissemination, 378; pro- 
jection, 383; waftage, 387; wings, 388; 
flotation, 393; vs. fruit, 391 

Seed sport s, 437 

Selection of variations, 429 

Self-planting, 400 

Semi-permeable membranes, 173 

Sense organs, 234 

Sensitive Plant, 237, 243, 244, 251 

Sensitivity, 145 

Sex, 278; meaning of, 286, 291; origin 

of, 288; prominence of, 292 
Sexual reproduction, significance, 286; 

superior to asexual, 293 
Sexual cells, 280, 288 
Shapes of leaves, 53 
Shoot and root explained, 49 
Sieve tubes, described, 155, 219, 221 
Skeleton of plants, 106, 107, 152, 257 
Sleep movements, 236, 238, 244, 266 
Sleeping rooms, plants in, 87 
Slime, or jelly, of plants, 273 
Slime molds, 141, 449 
Slowness of plant actions, 76 
Smilax, 67, 71 
Smuts, 273, 451 



Snails, in cross pollination, 324 

Society, plant, 465 

Soils, aeration, 85; structure, 86, 122; 
drainage, 87 

Spanish Moss, 455 

Special Creation, 403 

Spectroscope, described, 33 

Spermatozoids, 283, 290 

Spermatophytes, described, 459 

Sphagnum, 455 

Spines, 68; use, 275 

Spirogyra, 304, 446 

Spongiole, 169 

Spontaneity of variation, 432 

Spontaneous generation, 148 

Spores, colors, 263, asexual, 280, 281 ; in 
air, 391; 446, 453, 455, 457 

Sports, preservation, 435 

Spots, 451 

Sprengel, 314 

Spring, coloraton, 39; vegetation in, 368 

Spruce tree, 69 

Squirting Cucumber, 384, 386 

Stamens, sensitive, 323 

Starch, in leaves, 23, tests, 23, chemistry, 
27, 108, 110; iodine test, 109; as reserve 
food, 109; food for man, 109; grains, 
structure, 110, 112; digestion of, 110; 
individuality of, 111; in potato, 111 

Sterns, characteristics, 61; with func- 
tion of foliage, 70, 71; generalized, 
214; two types, 364; cellular anatomy, 
220, 221; construction, 222, 258 

Sterilization methods, 103 

Stigma, 281; sensitive, 323 

Stimulus, 229; perception of, 229; dif- 
ferential responses, 230, 231; action of, 
242, 247, 248, 253, 255; in growth, 
355, 358 

Stipa pinnata, 400 

Stipules, 53, 70 

Stolons, 381 

Stomata, 22; number, 207, size, 207; 
use of, 269, 273 

Stomatal chambers, 271 

Storage battery, 93, 96 

Strains, adjustments to, 252 

Strand plants, 465 



Index 



477 



Streaming of protoplasm, 140, 141 

Struggle for existence, 407 

Strychnine, 125 

Style, 281 

Suberin, 157 

Substances made by plants, 105 

Substitution foliage, 70 

Succulent plants, 269 

Suckers, 381 

Sucrose, 108 

Sugar, in leaves, 24, 27; various, 108 

Sulphur in plants, 120, 128 

Sulphur showers, 309 

Summer, vegetation in, 368 

Sundew, 245 

Super-vitalism, 14, 96 

Survival of fittest, 408 

Suspensor, 353 

Symmetry; in form, 238; 250; in growth, 

369 
Systematic Botany, defined, 2 

Tannins, 118, 274 

Taxis, 252 

Teleology, nature, 12 

Temperature, and growth, 332 

Tendrils, 67, 68, 243 

Thermonasty, 252 

Thermotropism, 251 

Thigmotropism, 242, 243 

Thought in nature, 147, 402 

Tissues, 157 

Toadstools, 451 

Tone, 253 

Torsions, 374 

Toxicodendrol, 1 17 

Traction, of water up stems, 216 

Trailers, 464 

Translocation of food, 217; in bark, 219 

Transmission of acquired characters, 
413 

Transpiration, described, 199; Experi- 
ment, 199; quantity, 200; determines 
phenomena, 202; variations in amount, 
203; effect of heat, 204, of light, 206, 
208, of dry ness, 206, of wind, 206; 
graph, 205; meaning, 209; and growth, 
335 



Transpirograph, 203, 204 
Traumatropism, 251 
Tree forms, 51, 58, 262 
Tree of ascent, 449 
Tropisms, 251 
Tulip Tree, stipules, 67 
Tumble-weeds, 392 
Tumors, 371 
Turf, 382 
Turgescence, 384 
Twin flower, 326, 397 
Twisted stems, 374 
Typical, meaning of, 9 

Undergrowth plants, 456, 460 

Unicorn Plant, 396 

Unit characters, 421 

Unity of science, 7 

Uroglama, 446 

Useless vs. useful science, 4 

Utility of science, 4, 5 

Vallisneria spiralis, 306 

Variations; nature, 407; selection of, 

429; experimental, 430; innate, 431; 

hereditary, 432; spontaneous, 432; 

fortuitous, 433 
Variegated plants, 436 
Vascular System, 458 
Vaucheria, 446 
Vegetable balls, 376 
Vegetable ivory, 112 
Vegetables, improvement of, 428 
Vegetative multiplication, 279 
Veins, 52, 63, 221 
Venation, 53 
Venus Flytrap, 245 
Verities; nature of, 9; 18, 24, 28, 35, 80, 

85, 92, 95, 103, 172, 176, 184 
Vertical position, 263 
Vetch, pods, 383 
Violet, flowers, 318; pods, 386 
Visualization of photosynthesis, 36; 

219, 251 

Vitalism, vii, viii, 13, 14, 96 
Vitality suspended, 164 
Volatile oils, 117 



Index 



Waftage of seeds, 387 

Walking Fern, 381 

Warping of wood, 187; diagram, 188 

Water, in photosynthesis, 30, 48; storage, 
269; protection against, 256, 271 

Water culture, methods, 136, 136 

Water-lily seed, 394 

Water-molds, 451 

Water-plants, aeration system, 195; 
448; 464 

Water-rolled balls, 376 

Waxes, 118 

Wearing out of varieties, 443 

Weismann, 415, 423 

Wild Geranium, 385 

Wind pollination, 307 

Winds, in pollination, 305; in dissemina- 
tion, 387; protection against, 256; 257 



Wings of seeds and fruits, 70, 388 
Winter-killing, 202 
Winter, vegetation in, 367 
Witches Brooms, 371 
Wooden flowers, 371 
Work of plants, 76, examples, 77; reality, 
79 

Xanthophyll, 41; in autumn leaves, 41 
X-rays, on plants, 251 
Xenia, 300 
Xerophytcs, 465 

Yeast, 97, 451 

Zoosporcs, 280, 380 
Zymase, 129